diff --git a/packages/overlayscrollbars/src/lifecycles/lifecycleHub.ts b/packages/overlayscrollbars/src/lifecycles/lifecycleHub.ts
index b1432d3..0b25eb8 100644
--- a/packages/overlayscrollbars/src/lifecycles/lifecycleHub.ts
+++ b/packages/overlayscrollbars/src/lifecycles/lifecycleHub.ts
@@ -1,28 +1,11 @@
-import {
-  XY,
-  WH,
-  TRBL,
-  CacheValues,
-  PartialOptions,
-  each,
-  hasOwnProperty,
-  isNumber,
-  scrollLeft,
-  scrollTop,
-  assignDeep,
-  liesBetween,
-  diffClass,
-} from 'support';
+import { XY, WH, TRBL, CacheValues, PartialOptions, each, hasOwnProperty, isNumber, scrollLeft, scrollTop, assignDeep } from 'support';
 import { OSOptions } from 'options';
-import { classNameHost, classNameViewport, classNameContent } from 'classnames';
 import { getEnvironment } from 'environment';
 import { StructureSetup } from 'setups/structureSetup';
+import { lifecycleHubOservers } from 'lifecycles/lifecycleHubObservers';
 import { createTrinsicLifecycle } from 'lifecycles/trinsicLifecycle';
 import { createPaddingLifecycle } from 'lifecycles/paddingLifecycle';
 import { createOverflowLifecycle } from 'lifecycles/overflowLifecycle';
-import { createSizeObserver } from 'observers/sizeObserver';
-import { createTrinsicObserver } from 'observers/trinsicObserver';
-import { createDOMObserver } from 'observers/domObserver';
 import { StyleObject } from 'typings';
 
 export type LifecycleCheckOption = <T>(path: string) => LifecycleOptionInfo<T>;
@@ -82,28 +65,7 @@ export interface LifecycleHub {
 const getPropByPath = <T>(obj: any, path: string): T =>
   obj ? path.split('.').reduce((o, prop) => (o && hasOwnProperty(o, prop) ? o[prop] : undefined), obj) : undefined;
 
-// TODO: observer textarea attrs if textarea
-// TODO: tabindex, open etc.
-// TODO: test _ignoreContentChange & _ignoreNestedTargetChange for content dom observer
-// TODO: test _ignoreTargetChange for target dom observer
-const ignorePrefix = 'os-';
-const hostSelector = `.${classNameHost}`;
-const viewportSelector = `.${classNameViewport}`;
-const contentSelector = `.${classNameContent}`;
-const attrs = ['id', 'class', 'style', 'open'];
-const ignoreTargetChange = (target: Node, attrName: string, oldValue: string | null, newValue: string | null) => {
-  if (attrName === 'class' && oldValue && newValue) {
-    const diff = diffClass(oldValue, newValue);
-    return !!diff.find((addedOrRemovedClass) => addedOrRemovedClass.indexOf(ignorePrefix) !== 0);
-  }
-  return false;
-};
-const directionIsRTLCacheValuesFallback: CacheValues<boolean> = {
-  _value: false,
-  _previous: false,
-  _changed: false,
-};
-const heightIntrinsicCacheValuesFallback: CacheValues<boolean> = {
+const booleanCacheValuesFallback: CacheValues<boolean> = {
   _value: false,
   _previous: false,
   _changed: false,
@@ -139,7 +101,7 @@ const lifecycleCommunicationFallback: LifecycleCommunication = {
 
 export const createLifecycleHub = (options: OSOptions, structureSetup: StructureSetup): LifecycleHubInstance => {
   let lifecycleCommunication = lifecycleCommunicationFallback;
-  const { _host, _viewport, _content } = structureSetup._targetObj;
+  const { _viewport } = structureSetup._targetObj;
   const {
     _nativeScrollbarStyling,
     _nativeScrollbarIsOverlaid,
@@ -168,10 +130,11 @@ export const createLifecycleHub = (options: OSOptions, structureSetup: Structure
       _contentMutation = force || false,
       _paddingStyleChanged = force || false,
     } = updateHints || {};
+
     const finalDirectionIsRTL =
-      _directionIsRTL || (sizeObserver ? sizeObserver._getCurrentCacheValues(force)._directionIsRTL : directionIsRTLCacheValuesFallback);
+      _directionIsRTL || (_sizeObserver ? _sizeObserver._getCurrentCacheValues(force)._directionIsRTL : booleanCacheValuesFallback);
     const finalHeightIntrinsic =
-      _heightIntrinsic || (trinsicObserver ? trinsicObserver._getCurrentCacheValues(force)._heightIntrinsic : heightIntrinsicCacheValuesFallback);
+      _heightIntrinsic || (_trinsicObserver ? _trinsicObserver._getCurrentCacheValues(force)._heightIntrinsic : booleanCacheValuesFallback);
     const checkOption: LifecycleCheckOption = (path) => ({
       _value: getPropByPath(options, path),
       _changed: force || getPropByPath(changedOptions, path) !== undefined,
@@ -180,6 +143,11 @@ export const createLifecycleHub = (options: OSOptions, structureSetup: Structure
     const scrollOffsetX = adjustScrollOffset && scrollLeft(_viewport);
     const scrollOffsetY = adjustScrollOffset && scrollTop(_viewport);
 
+    // place before updating lifecycles because of possible flushing of debounce
+    if (_updateObserverOptions) {
+      _updateObserverOptions(checkOption);
+    }
+
     each(lifecycles, (lifecycle) => {
       const {
         _sizeChanged: adaptiveSizeChanged,
@@ -217,58 +185,7 @@ export const createLifecycleHub = (options: OSOptions, structureSetup: Structure
       options.callbacks.onUpdated();
     }
   };
-
-  const onSizeChanged = (directionIsRTL?: CacheValues<boolean>) => {
-    const sizeChanged = !directionIsRTL;
-    updateLifecycles({
-      _directionIsRTL: directionIsRTL,
-      _sizeChanged: sizeChanged,
-    });
-  };
-  const onTrinsicChanged = (heightIntrinsic: CacheValues<boolean>) => {
-    updateLifecycles({
-      _heightIntrinsic: heightIntrinsic,
-    });
-  };
-  const onHostMutation = () => {
-    // TODO: rAF only here because IE
-    requestAnimationFrame(() => {
-      updateLifecycles({
-        _hostMutation: true,
-      });
-    });
-  };
-  const onContentMutation = () => {
-    // TODO: rAF only here because IE
-    requestAnimationFrame(() => {
-      updateLifecycles({
-        _contentMutation: true,
-      });
-    });
-  };
-
-  const trinsicObserver = (_content || !_flexboxGlue) && createTrinsicObserver(_host, onTrinsicChanged);
-  const sizeObserver = createSizeObserver(_host, onSizeChanged, { _appear: true, _direction: !_nativeScrollbarStyling });
-  const hostMutationObserver = createDOMObserver(_host, false, onHostMutation, {
-    _styleChangingAttributes: attrs,
-    _attributes: attrs,
-    _ignoreTargetChange: ignoreTargetChange,
-  });
-  const contentMutationObserver = createDOMObserver(_content || _viewport, true, onContentMutation, {
-    _styleChangingAttributes: attrs,
-    _attributes: attrs,
-    _eventContentChange: options!.updating!.elementEvents,
-    _nestedTargetSelector: hostSelector,
-    _ignoreContentChange: (mutation, isNestedTarget) => {
-      const { target, attributeName } = mutation;
-      return isNestedTarget
-        ? false
-        : attributeName
-        ? liesBetween(target as Element, hostSelector, viewportSelector) || liesBetween(target as Element, hostSelector, contentSelector)
-        : false;
-    },
-    _ignoreNestedTargetChange: ignoreTargetChange,
-  });
+  const { _sizeObserver, _trinsicObserver, _updateObserverOptions } = lifecycleHubOservers(instance, updateLifecycles);
 
   const update = (changedOptions?: Partial<OSOptions> | null, force?: boolean) => {
     updateLifecycles(null, changedOptions, force);
diff --git a/packages/overlayscrollbars/src/lifecycles/lifecycleHubObservers.ts b/packages/overlayscrollbars/src/lifecycles/lifecycleHubObservers.ts
new file mode 100644
index 0000000..9b13dd0
--- /dev/null
+++ b/packages/overlayscrollbars/src/lifecycles/lifecycleHubObservers.ts
@@ -0,0 +1,141 @@
+import { CacheValues, diffClass, debounce, isArray, isNumber } from 'support';
+import { getEnvironment } from 'environment';
+import { createSizeObserver, SizeObserverCallbackParams } from 'observers/sizeObserver';
+import { createTrinsicObserver } from 'observers/trinsicObserver';
+import { createDOMObserver, DOMObserver } from 'observers/domObserver';
+import { LifecycleHub, LifecycleCheckOption, LifecycleUpdateHints } from 'lifecycles/lifecycleHub';
+
+//const hostSelector = `.${classNameHost}`;
+
+// TODO: observer textarea attrs if textarea
+// TODO: tabindex, etc. attributes for viewport
+// TODO: test _ignoreContentChange & _ignoreNestedTargetChange for content dom observer
+// TODO: test _ignoreTargetChange for target dom observer
+
+//const viewportSelector = `.${classNameViewport}`;
+//const contentSelector = `.${classNameContent}`;
+const ignorePrefix = 'os-';
+const viewportAttrsFromTarget = ['tabindex'];
+const baseStyleChangingAttrsTextarea = ['wrap', 'cols', 'rows'];
+const baseStyleChangingAttrs = ['id', 'class', 'style', 'open'];
+
+const ignoreTargetChange = (target: Node, attrName: string, oldValue: string | null, newValue: string | null) => {
+  if (attrName === 'class' && oldValue && newValue) {
+    const diff = diffClass(oldValue, newValue);
+    return !!diff.find((addedOrRemovedClass) => addedOrRemovedClass.indexOf(ignorePrefix) !== 0);
+  }
+  return false;
+};
+
+export const lifecycleHubOservers = (instance: LifecycleHub, updateLifecycles: (updateHints?: Partial<LifecycleUpdateHints> | null) => unknown) => {
+  let debounceTimeout: number | false | undefined;
+  let debounceMaxDelay: number | false | undefined;
+  const { _structureSetup } = instance;
+  const { _targetObj, _targetCtx } = _structureSetup;
+  const { _host, _viewport, _content } = _targetObj;
+  const { _isTextarea } = _targetCtx;
+  const { _nativeScrollbarStyling, _flexboxGlue } = getEnvironment();
+  const contentMutationObserverAttr = _isTextarea ? baseStyleChangingAttrsTextarea : baseStyleChangingAttrs.concat(baseStyleChangingAttrsTextarea);
+  const updateLifecyclesWithDebouncedAdaptiveUpdateHints = debounce(updateLifecycles as (updateHints: Partial<LifecycleUpdateHints>) => any, {
+    _timeout: () => debounceTimeout,
+    _maxDelay: () => debounceMaxDelay,
+    _mergeParams(prev, curr) {
+      const { _sizeChanged: prevSizeChanged, _hostMutation: prevHostMutation, _contentMutation: prevContentMutation } = prev[0];
+      const { _sizeChanged: currSizeChanged, _hostMutation: currvHostMutation, _contentMutation: currContentMutation } = curr[0];
+      const merged: [Partial<LifecycleUpdateHints>] = [
+        {
+          _sizeChanged: prevSizeChanged || currSizeChanged,
+          _hostMutation: prevHostMutation || currvHostMutation,
+          _contentMutation: prevContentMutation || currContentMutation,
+        },
+      ];
+
+      return merged;
+    },
+  });
+
+  const onTrinsicChanged = (heightIntrinsic: CacheValues<boolean>) => {
+    updateLifecycles({
+      _heightIntrinsic: heightIntrinsic,
+    });
+  };
+  const onSizeChanged = ({ _sizeChanged, _directionIsRTLCache, _appear }: SizeObserverCallbackParams) => {
+    const updateFn = !_sizeChanged || _appear ? updateLifecycles : updateLifecyclesWithDebouncedAdaptiveUpdateHints;
+    updateFn({
+      _sizeChanged,
+      _directionIsRTL: _directionIsRTLCache,
+    });
+  };
+  const onContentMutation = (contentChangedTroughEvent: boolean) => {
+    // if contentChangedTroughEvent is true its already debounced
+    const updateFn = contentChangedTroughEvent ? updateLifecycles : updateLifecyclesWithDebouncedAdaptiveUpdateHints;
+    updateFn({
+      _contentMutation: true,
+    });
+  };
+  const onHostMutation = updateLifecyclesWithDebouncedAdaptiveUpdateHints.bind(0, {
+    _hostMutation: true,
+  }) as () => any;
+
+  const trinsicObserver = (_content || !_flexboxGlue) && createTrinsicObserver(_host, onTrinsicChanged);
+  const sizeObserver = createSizeObserver(_host, onSizeChanged, { _appear: true, _direction: !_nativeScrollbarStyling });
+  const hostMutationObserver = createDOMObserver(_host, false, onHostMutation, {
+    _styleChangingAttributes: baseStyleChangingAttrs,
+    _attributes: baseStyleChangingAttrs,
+    _ignoreTargetChange: ignoreTargetChange,
+  });
+  let contentMutationObserver: DOMObserver | undefined;
+
+  const updateOptions = (checkOption: LifecycleCheckOption) => {
+    const { _value: elementEvents, _changed: elementEventsChanged } = checkOption<Array<[string, string]> | null>('updating.elementEvents');
+    const { _value: attributes, _changed: attributesChanged } = checkOption<string[] | null>('updating.attributes');
+    const { _value: debounce, _changed: debounceChanged } = checkOption<Array<number> | number | null>('updating.debounce');
+    const updateContentMutationObserver = elementEventsChanged || attributesChanged;
+
+    if (updateContentMutationObserver) {
+      if (contentMutationObserver) {
+        contentMutationObserver._update();
+        contentMutationObserver._destroy();
+      }
+      contentMutationObserver = createDOMObserver(_content || _viewport, true, onContentMutation, {
+        _styleChangingAttributes: contentMutationObserverAttr.concat(attributes || []),
+        _attributes: contentMutationObserverAttr.concat(attributes || []),
+        _eventContentChange: elementEvents,
+        _ignoreNestedTargetChange: ignoreTargetChange,
+        //_nestedTargetSelector: hostSelector,
+        /*
+        _ignoreContentChange: (mutation, isNestedTarget) => {
+          const { target, attributeName } = mutation;
+          return isNestedTarget
+            ? false
+            : attributeName
+            ? liesBetween(target as Element, hostSelector, viewportSelector) || liesBetween(target as Element, hostSelector, contentSelector)
+            : false;
+        },
+        */
+      });
+    }
+
+    if (debounceChanged) {
+      updateLifecyclesWithDebouncedAdaptiveUpdateHints._flush();
+      if (isArray(debounce)) {
+        const timeout = debounce[0];
+        const maxWait = debounce[1];
+        debounceTimeout = isNumber(timeout) ? timeout : false;
+        debounceMaxDelay = isNumber(maxWait) ? maxWait : false;
+      } else if (isNumber(debounce)) {
+        debounceTimeout = debounce;
+        debounceMaxDelay = false;
+      } else {
+        debounceTimeout = false;
+        debounceMaxDelay = false;
+      }
+    }
+  };
+
+  return {
+    _trinsicObserver: trinsicObserver,
+    _sizeObserver: sizeObserver,
+    _updateObserverOptions: updateOptions,
+  };
+};
diff --git a/packages/overlayscrollbars/src/observers/domObserver.ts b/packages/overlayscrollbars/src/observers/domObserver.ts
index 6674f4a..fc826a3 100644
--- a/packages/overlayscrollbars/src/observers/domObserver.ts
+++ b/packages/overlayscrollbars/src/observers/domObserver.ts
@@ -13,12 +13,11 @@ import {
   find,
   push,
   isUndefined,
-  isFunction,
 } from 'support';
 
 type StringNullUndefined = string | null | undefined;
 
-type DOMContentObserverCallback = (contentChanged: boolean) => any;
+type DOMContentObserverCallback = (contentChangedTroughEvent: boolean) => any;
 
 type DOMTargetObserverCallback = (targetChangedAttrs: string[], targetStyleChanged: boolean) => any;
 
@@ -38,18 +37,7 @@ interface DOMTargetObserverOptions extends DOMObserverOptionsBase {
   _ignoreTargetChange?: DOMObserverIgnoreTargetChange; // a function which will prevent marking certain attributes as changed if it returns true
 }
 
-interface DOMObserverBase {
-  _destroy: () => void;
-  _update: () => void;
-}
-
-interface DOMContentObserver extends DOMObserverBase {
-  _updateEventContentChange: (newEventContentChange?: DOMObserverEventContentChange) => void;
-}
-
-interface DOMTargetObserver extends DOMObserverBase {}
-
-type ContentChangeArrayItem = [StringNullUndefined, ((elms: Node[]) => StringNullUndefined) | StringNullUndefined] | null | undefined;
+type ContentChangeArrayItem = [StringNullUndefined, StringNullUndefined] | null | undefined;
 
 export type DOMObserverEventContentChange = Array<ContentChangeArrayItem> | false | null | undefined;
 
@@ -57,7 +45,7 @@ export type DOMObserverIgnoreContentChange = (
   mutation: MutationRecord,
   isNestedTarget: boolean,
   domObserverTarget: HTMLElement,
-  domObserverOptions: DOMContentObserverOptions | undefined
+  domObserverOptions?: DOMContentObserverOptions
 ) => boolean;
 
 export type DOMObserverIgnoreTargetChange = (
@@ -73,10 +61,10 @@ export type DOMObserverCallback<ContentObserver extends boolean> = ContentObserv
 
 export type DOMObserverOptions<ContentObserver extends boolean> = ContentObserver extends true ? DOMContentObserverOptions : DOMTargetObserverOptions;
 
-export type DOMObserver<ContentObserver extends boolean> = ContentObserver extends true ? DOMContentObserver : DOMTargetObserver;
-
-// const styleChangingAttributes = ['id', 'class', 'style', 'open'];
-// const mutationObserverAttrsTextarea = ['wrap', 'cols', 'rows'];
+export interface DOMObserver {
+  _destroy: () => void;
+  _update: () => void;
+}
 
 /**
  * Creates a set of helper functions to observe events of elements inside the target element.
@@ -87,7 +75,6 @@ export type DOMObserver<ContentObserver extends boolean> = ContentObserver exten
  */
 const createEventContentChange = (target: Element, eventContentChange: DOMObserverEventContentChange, callback: (...args: any) => any) => {
   let map: Map<Node, string> | undefined;
-  let eventContentChangeRef: DOMObserverEventContentChange;
   const _destroy = () => {
     if (map) {
       map.forEach((eventName: string, elm: Node) => off(elm, eventName, callback));
@@ -95,16 +82,15 @@ const createEventContentChange = (target: Element, eventContentChange: DOMObserv
     }
   };
   const _updateElements = (getElements?: (selector: string) => Node[]) => {
-    if (map && eventContentChangeRef) {
-      const eventElmList = eventContentChangeRef.reduce<Array<[Node[], string]>>((arr, item) => {
+    if (map && eventContentChange) {
+      const eventElmList = eventContentChange.reduce<Array<[Node[], string]>>((arr, item) => {
         if (item) {
           const selector = item[0];
           const eventNames = item[1];
           const elements = eventNames && selector && (getElements ? getElements(selector) : find(selector, target));
-          const parsedEventNames = isFunction(eventNames) ? eventNames(elements) : eventNames;
 
-          if (elements && elements.length && parsedEventNames && isString(parsedEventNames)) {
-            push(arr, [elements, parsedEventNames.trim()], true);
+          if (elements && elements.length && eventNames && isString(eventNames)) {
+            push(arr, [elements, eventNames.trim()], true);
           }
         }
         return arr;
@@ -128,21 +114,16 @@ const createEventContentChange = (target: Element, eventContentChange: DOMObserv
       );
     }
   };
-  const _updateEventContentChange = (newEventContentChange: DOMObserverEventContentChange) => {
-    map = map || new Map<Node, string>();
-    eventContentChangeRef = newEventContentChange;
-    _destroy();
-    _updateElements();
-  };
 
   if (eventContentChange) {
-    _updateEventContentChange(eventContentChange);
+    map = map || new Map<Node, string>();
+    _destroy();
+    _updateElements();
   }
 
   return {
     _destroy,
     _updateElements,
-    _updateEventContentChange,
   };
 };
 
@@ -159,7 +140,7 @@ export const createDOMObserver = <ContentObserver extends boolean>(
   isContentObserver: ContentObserver,
   callback: DOMObserverCallback<ContentObserver>,
   options?: DOMObserverOptions<ContentObserver>
-): DOMObserver<ContentObserver> => {
+): DOMObserver => {
   let isConnected = false;
   const {
     _attributes,
@@ -170,18 +151,17 @@ export const createDOMObserver = <ContentObserver extends boolean>(
     _ignoreNestedTargetChange,
     _ignoreContentChange,
   } = (options as DOMContentObserverOptions & DOMTargetObserverOptions) || {};
-  const {
-    _destroy: destroyEventContentChange,
-    _updateElements: updateEventContentChangeElements,
-    _updateEventContentChange: updateEventContentChange,
-  } = createEventContentChange(
+  const { _destroy: destroyEventContentChange, _updateElements: updateEventContentChangeElements } = createEventContentChange(
     target,
     isContentObserver && _eventContentChange,
-    debounce(() => {
-      if (isConnected) {
-        (callback as DOMContentObserverCallback)(true);
-      }
-    }, 84)
+    debounce(
+      () => {
+        if (isConnected) {
+          (callback as DOMContentObserverCallback)(true);
+        }
+      },
+      { _timeout: 33, _maxDelay: 99 }
+    )
   );
 
   // MutationObserver
@@ -243,7 +223,7 @@ export const createDOMObserver = <ContentObserver extends boolean>(
     }
 
     if (isContentObserver) {
-      contentChanged && (callback as DOMContentObserverCallback)(contentChanged);
+      contentChanged && (callback as DOMContentObserverCallback)(false);
     } else if (!isEmptyArray(targetChangedAttrs) || targetStyleChanged) {
       (callback as DOMTargetObserverCallback)(targetChangedAttrs, targetStyleChanged);
     }
@@ -269,13 +249,10 @@ export const createDOMObserver = <ContentObserver extends boolean>(
         isConnected = false;
       }
     },
-    _updateEventContentChange: (newEventContentChange?: DOMObserverEventContentChange) => {
-      updateEventContentChange(isConnected && isContentObserver && newEventContentChange);
-    },
     _update: () => {
       if (isConnected) {
         observerCallback(mutationObserver.takeRecords());
       }
     },
-  } as DOMObserver<ContentObserver>;
+  };
 };
diff --git a/packages/overlayscrollbars/src/observers/sizeObserver.ts b/packages/overlayscrollbars/src/observers/sizeObserver.ts
index 882fa9c..b8d7dd4 100644
--- a/packages/overlayscrollbars/src/observers/sizeObserver.ts
+++ b/packages/overlayscrollbars/src/observers/sizeObserver.ts
@@ -33,7 +33,16 @@ import {
   classNameSizeObserverListenerItemFinal,
 } from 'classnames';
 
-export type SizeObserverOptions = { _direction?: boolean; _appear?: boolean };
+export interface SizeObserverOptions {
+  _direction?: boolean;
+  _appear?: boolean;
+}
+
+export interface SizeObserverCallbackParams {
+  _sizeChanged: boolean;
+  _directionIsRTLCache?: CacheValues<boolean>;
+  _appear?: boolean;
+}
 
 export interface SizeObserver {
   _destroy(): void;
@@ -73,7 +82,7 @@ const domRectHasDimensions = (rect?: DOMRectReadOnly) => rect && (rect.height ||
  */
 export const createSizeObserver = (
   target: HTMLElement,
-  onSizeChangedCallback: (directionIsRTLCache?: CacheValues<boolean>) => any,
+  onSizeChangedCallback: (params: SizeObserverCallbackParams) => any,
   options?: SizeObserverOptions
 ): SizeObserver => {
   const { _direction: observeDirectionChange = false, _appear: observeAppearChange = false } = options || {};
@@ -90,31 +99,44 @@ export const createSizeObserver = (
         (!domRectHasDimensions(currVal) && domRectHasDimensions(newVal))
       ),
   });
-  const onSizeChangedCallbackProxy = (sizeChangedContext?: CacheValues<boolean> | ResizeObserverEntry[] | Event) => {
+  const onSizeChangedCallbackProxy = (sizeChangedContext?: CacheValues<boolean> | ResizeObserverEntry[] | Event | boolean) => {
     const hasDirectionCache = sizeChangedContext && isBoolean((sizeChangedContext as CacheValues<boolean>)._value);
 
     let skip = false;
+    let appear: boolean | number | undefined = false;
     let doDirectionScroll = true; // always true if sizeChangedContext is Event (appear callback or RO. Polyfill)
 
     // if triggered from RO.
     if (isArray(sizeChangedContext) && sizeChangedContext.length > 0) {
-      const { _previous, _value, _changed } = updateResizeObserverContentRectCache(0, sizeChangedContext.pop()!.contentRect);
-      skip = !_previous || !domRectHasDimensions(_value); // skip on initial RO. call or if display is none
-      doDirectionScroll = !skip && _changed; // direction scroll when not skipping and changing from display: none to block, false otherwise
+      const { _previous, _value } = updateResizeObserverContentRectCache(0, sizeChangedContext.pop()!.contentRect);
+      const hasDimensions = domRectHasDimensions(_value);
+      const hadDimensions = domRectHasDimensions(_previous);
+      skip = !_previous || !hasDimensions; // skip on initial RO. call or if display is none
+      appear = !hadDimensions && hasDimensions;
+
+      doDirectionScroll = !skip; // direction scroll when not skipping
     }
     // else if its triggered with DirectionCache
     else if (hasDirectionCache) {
       doDirectionScroll = (sizeChangedContext as CacheValues<boolean>)._changed; // direction scroll when DirectionCache changed, false otherwise
     }
+    // else if it triggered with appear from polyfill
+    else {
+      appear = sizeChangedContext === true;
+    }
 
-    if (observeDirectionChange) {
+    if (observeDirectionChange && doDirectionScroll) {
       const rtl = hasDirectionCache ? (sizeChangedContext as CacheValues<boolean>)._value : directionIsRTL(sizeObserver);
       scrollLeft(sizeObserver, rtl ? (rtlScrollBehavior.n ? -scrollAmount : rtlScrollBehavior.i ? 0 : scrollAmount) : scrollAmount);
       scrollTop(sizeObserver, scrollAmount);
     }
 
     if (!skip) {
-      onSizeChangedCallback(hasDirectionCache ? (sizeChangedContext as CacheValues<boolean>) : undefined);
+      onSizeChangedCallback({
+        _sizeChanged: !hasDirectionCache,
+        _directionIsRTLCache: hasDirectionCache ? (sizeChangedContext as CacheValues<boolean>) : undefined,
+        _appear: !!appear,
+      });
     }
   };
   const offListeners: (() => void)[] = [];
@@ -147,11 +169,11 @@ export const createSizeObserver = (
       scrollLeft(shrinkElement, scrollAmount);
       scrollTop(shrinkElement, scrollAmount);
     };
-    const onResized = () => {
+    const onResized = (appear?: unknown) => {
       rAFId = 0;
       if (isDirty) {
         cacheSize = currSize;
-        onSizeChangedCallbackProxy();
+        onSizeChangedCallbackProxy(appear === true);
       }
     };
     const onScroll = (scrollEvent?: Event | false) => {
@@ -166,7 +188,7 @@ export const createSizeObserver = (
           rAFId = rAF!(onResized);
         }
       } else {
-        onResized();
+        onResized(scrollEvent === false);
       }
 
       reset();
diff --git a/packages/overlayscrollbars/src/options.ts b/packages/overlayscrollbars/src/options.ts
index 0ca69f7..5c77b14 100644
--- a/packages/overlayscrollbars/src/options.ts
+++ b/packages/overlayscrollbars/src/options.ts
@@ -35,9 +35,8 @@ export interface OSOptions {
   paddingAbsolute: boolean;
   updating: {
     elementEvents: Array<[string, string]> | null;
-    contentMutationDebounce: number;
-    hostMutationDebounce: number;
-    resizeDebounce: number;
+    attributes: string[] | null;
+    debounce: number | [number, number] | null;
   };
   overflow: {
     x: OverflowBehavior;
@@ -109,6 +108,7 @@ export interface UpdatedArgs {
 }
 
 const numberAllowedValues: OptionsTemplateValue<number> = oTypes.number;
+const arrayNullValues: OptionsTemplateValue<Array<unknown> | null> = [oTypes.array, oTypes.null];
 const stringArrayNullAllowedValues: OptionsTemplateValue<string | ReadonlyArray<string> | null> = [oTypes.string, oTypes.array, oTypes.null];
 const booleanTrueTemplate: OptionsWithOptionsTemplateValue<boolean> = [true, oTypes.boolean];
 const booleanFalseTemplate: OptionsWithOptionsTemplateValue<boolean> = [false, oTypes.boolean];
@@ -137,10 +137,12 @@ const defaultOptionsWithTemplate: OptionsWithOptionsTemplate<OSOptions> = {
   resize: ['none', resizeAllowedValues], // none || both  || horizontal || vertical || n || b || h || v
   paddingAbsolute: booleanFalseTemplate, // true || false
   updating: {
-    elementEvents: [[['img', 'load']], [oTypes.array, oTypes.null]], // array of tuples || null
-    contentMutationDebounce: [80, numberAllowedValues], // number
-    hostMutationDebounce: [0, numberAllowedValues], // number
-    resizeDebounce: [0, numberAllowedValues], // number
+    elementEvents: [[['img', 'load']], arrayNullValues], // array of tuples || null
+    attributes: [null, arrayNullValues],
+    debounce: [
+      [0, 33],
+      [oTypes.number, oTypes.array, oTypes.null],
+    ], // number || number array || null
   },
   overflow: {
     x: ['scroll', overflowAllowedValues], // visible-hidden  || visible-scroll || hidden || scroll || v-h || v-s || h || s
diff --git a/packages/overlayscrollbars/src/support/utils/function.ts b/packages/overlayscrollbars/src/support/utils/function.ts
index c5d7f8f..d0925ad 100644
--- a/packages/overlayscrollbars/src/support/utils/function.ts
+++ b/packages/overlayscrollbars/src/support/utils/function.ts
@@ -1,36 +1,103 @@
-import { isNumber } from 'support/utils/types';
-import { cAF, rAF } from 'support/compatibility/apis';
+import { isNumber, isFunction } from 'support/utils/types';
+import { rAF, cAF } from 'support/compatibility/apis';
+
+const setT = window.setTimeout;
+const clearTimeouts = (id: number | undefined) => {
+  id && window.clearTimeout(id);
+  id && cAF!(id);
+};
+
+type DebounceTiming = number | false | null | undefined;
+
+export interface DebounceOptions<FunctionToDebounce extends (...args: any) => any> {
+  /**
+   * The timeout for debouncing. If null, no debounce is applied.
+   */
+  _timeout?: DebounceTiming | (() => DebounceTiming);
+  /**
+   * A maximum amount of ms. before the function will be called even with debounce.
+   */
+  _maxDelay?: DebounceTiming | (() => DebounceTiming);
+  /**
+   * Function which merges parameters for each canceled debounce.
+   * If parameters can't be merged the function will return null, otherwise it returns the merged parameters.
+   */
+  _mergeParams?: (
+    prev: Parameters<FunctionToDebounce>,
+    curr: Parameters<FunctionToDebounce>
+  ) => Parameters<FunctionToDebounce> | false | null | undefined;
+}
+
+export interface Debounced<FunctionToDebounce extends (...args: any) => any> {
+  (...args: Parameters<FunctionToDebounce>): ReturnType<FunctionToDebounce>;
+  _flush(): void;
+}
 
 export const noop = () => {}; // eslint-disable-line
 
 /**
  * Debounces the given function either with a timeout or a animation frame.
  * @param functionToDebounce The function which shall be debounced.
- * @param timeout The timeout for debouncing. If 0 or lower animation frame is used for debouncing, a timeout otherwise.
- * @param maxWait A maximum amount of ms. before the function will be called even with debounce.
+ * @param options Options for debouncing.
  */
-export const debounce = (functionToDebounce: (...args: any) => any, timeout?: number, maxWait?: number) => {
-  let timeoutId: number | void;
-  let lastCallTime: number;
-  const hasTimeout = isNumber(timeout) && timeout > 0;
-  const hasMaxWait = isNumber(maxWait) && maxWait > 0;
-  const cancel = hasTimeout ? window.clearTimeout : cAF!;
-  const set = hasTimeout ? window.setTimeout : rAF!;
-  const setFn = function (args: IArguments) {
-    lastCallTime = hasMaxWait ? performance.now() : 0;
-    timeoutId && cancel(timeoutId);
+export const debounce = <FunctionToDebounce extends (...args: any) => any>(
+  functionToDebounce: FunctionToDebounce,
+  options: DebounceOptions<FunctionToDebounce>
+): Debounced<FunctionToDebounce> => {
+  let timeoutId: number | undefined;
+  let maxTimeoutId: number | undefined;
+  let prevArguments: Parameters<FunctionToDebounce> | null | undefined;
+  let latestArguments: Parameters<FunctionToDebounce> | null | undefined;
+  const { _timeout, _maxDelay, _mergeParams } = options;
+
+  const invokeFunctionToDebounce = function (args: IArguments) {
+    clearTimeouts(timeoutId);
+    clearTimeouts(maxTimeoutId);
+    maxTimeoutId = timeoutId = prevArguments = undefined;
     // eslint-disable-next-line
     // @ts-ignore
     functionToDebounce.apply(this, args);
   };
 
-  return function () {
-    // eslint-disable-next-line
-    // @ts-ignore
-    const boundSetFn = setFn.bind(this, arguments); // eslint-disable-line
-    const forceCall = hasMaxWait ? performance.now() - lastCallTime >= maxWait! : false;
+  const mergeParms = (curr: Parameters<FunctionToDebounce>): Parameters<FunctionToDebounce> | false | null | undefined =>
+    _mergeParams && prevArguments ? _mergeParams(prevArguments, curr) : curr;
 
-    timeoutId && cancel(timeoutId);
-    timeoutId = forceCall ? boundSetFn() : (set(boundSetFn, timeout!) as number);
+  const flush = () => {
+    if (timeoutId) {
+      invokeFunctionToDebounce(mergeParms(latestArguments!) || latestArguments!);
+    }
   };
+
+  const debouncedFn = function () {
+    const args: Parameters<FunctionToDebounce> = arguments as Parameters<FunctionToDebounce>;
+    const finalTimeout = isFunction(_timeout) ? _timeout() : _timeout;
+    const hasTimeout = isNumber(finalTimeout) && finalTimeout >= 0;
+
+    if (hasTimeout) {
+      const finalMaxWait = isFunction(_maxDelay) ? _maxDelay() : _maxDelay;
+      const hasMaxWait = isNumber(finalMaxWait) && finalMaxWait >= 0;
+      const setTimeoutFn = finalTimeout! > 0 ? setT : rAF!;
+      const mergeParamsResult = mergeParms(args);
+      const invokedArgs = mergeParamsResult || args;
+      const boundInvoke = invokeFunctionToDebounce.bind(0, invokedArgs);
+
+      if (!mergeParamsResult) {
+        invokeFunctionToDebounce(prevArguments || args);
+      }
+
+      clearTimeouts(timeoutId);
+      timeoutId = setTimeoutFn(boundInvoke, finalTimeout as number) as number;
+
+      if (hasMaxWait && !maxTimeoutId) {
+        maxTimeoutId = setT(flush, finalMaxWait as number);
+      }
+
+      prevArguments = latestArguments = invokedArgs;
+    } else {
+      invokeFunctionToDebounce(args);
+    }
+  };
+  debouncedFn._flush = flush;
+
+  return debouncedFn as Debounced<FunctionToDebounce>;
 };
diff --git a/packages/overlayscrollbars/tests/browser/observers/domObserver/index.browser.ts b/packages/overlayscrollbars/tests/browser/observers/domObserver/index.browser.ts
index 44ef84c..a9b31b1 100644
--- a/packages/overlayscrollbars/tests/browser/observers/domObserver/index.browser.ts
+++ b/packages/overlayscrollbars/tests/browser/observers/domObserver/index.browser.ts
@@ -4,24 +4,14 @@ import should from 'should';
 import { generateSelectCallback, iterateSelect } from '@/testing-browser/Select';
 import { timeout } from '@/testing-browser/timeout';
 import { setTestResult, waitForOrFailTest } from '@/testing-browser/TestResult';
-import {
-  appendChildren,
-  createDiv,
-  removeElements,
-  children,
-  isArray,
-  isNumber,
-  liesBetween,
-  hasClass,
-  addClass,
-  removeClass,
-  diffClass,
-  on,
-} from 'support';
+import { appendChildren, createDiv, removeElements, children, isArray, isNumber, liesBetween, addClass, removeClass, diffClass, on } from 'support';
 
 import { createDOMObserver } from 'observers/domObserver';
 
-type DOMContentObserverResult = boolean;
+type DOMContentObserverResult = {
+  contentChange: boolean;
+  troughEvent: boolean;
+};
 type DOMTargetObserverResult = {
   changedTargetAttrs: string[];
   styleChanged: boolean;
@@ -34,6 +24,7 @@ interface SeparateChangeThrough {
 const targetChangesCountSlot: HTMLElement | null = document.querySelector('#targetChanges');
 const contentChangesCountSlot: HTMLElement | null = document.querySelector('#contentChanges');
 const targetElm: HTMLElement | null = document.querySelector('#target');
+const trargetContentElm: HTMLElement | null = document.querySelector('#target .content');
 const targetElmContentElm: HTMLElement | null = document.querySelector('#content-host');
 const contentElmAttrChange: HTMLElement | null = document.querySelector('#target .content-nest');
 const contentBetweenElmAttrChange: HTMLElement | null = document.querySelector('#content-host .padding-nest-item');
@@ -67,7 +58,7 @@ const startBtn: HTMLButtonElement | null = document.querySelector('#start');
 const hostSelector = '.host';
 const ignorePrefix = 'ignore';
 const attrs = ['id', 'class', 'style', 'open'];
-const contentChangeArr: Array<[string, string | ((elms: Node[]) => string)]> = [['img', 'load']];
+const contentChangeArr: Array<[string, string]> = [['img', 'load']];
 const domTargetObserverObservations: DOMTargetObserverResult[] = [];
 const domContentObserverObservations: DOMContentObserverResult[] = [];
 
@@ -115,44 +106,48 @@ const targetDomObserver = createDOMObserver(
   }
 );
 
-const contentDomObserver = createDOMObserver(
-  document.querySelector('#target .content')!,
-  true,
-  (contentChanged: boolean) => {
-    should.equal(typeof contentChanged, 'boolean', 'The contentChanged parameter in a content dom observer must be a boolean.');
+const createContentDomOserver = (eventContentChange: Array<[string | null | undefined, string | null | undefined] | null | undefined>) => {
+  return createDOMObserver(
+    trargetContentElm!,
+    true,
+    (contentChangedTroughEvent: boolean) => {
+      should.equal(typeof contentChangedTroughEvent, 'boolean', 'The contentChanged parameter in a content dom observer must be a boolean.');
 
-    domContentObserverObservations.push(contentChanged);
-    requestAnimationFrame(() => {
-      if (contentChangesCountSlot) {
-        contentChangesCountSlot.textContent = `${domContentObserverObservations.length}`;
-      }
-    });
-  },
-  {
-    _styleChangingAttributes: attrs,
-    _attributes: attrs,
-    _eventContentChange: contentChangeArr,
-    _nestedTargetSelector: hostSelector,
-    _ignoreContentChange: (mutation, isNestedTarget) => {
-      const { target, attributeName } = mutation;
-      return isNestedTarget ? false : attributeName ? liesBetween(target as Element, hostSelector, '.content') : false;
+      domContentObserverObservations.push({ contentChange: true, troughEvent: contentChangedTroughEvent });
+      requestAnimationFrame(() => {
+        if (contentChangesCountSlot) {
+          contentChangesCountSlot.textContent = `${domContentObserverObservations.length}`;
+        }
+      });
     },
-    _ignoreNestedTargetChange: (target, attrName, oldValue, newValue) => {
-      if (attrName === 'class' && oldValue && newValue) {
-        const diff = diffClass(oldValue, newValue);
-        const ignore = diff.length === 1 && diff[0].startsWith(ignorePrefix);
-        return ignore;
-      }
-      return false;
-    },
-    // @ts-ignore
-    _ignoreTargetChange: () => {
-      // if param: isContentObserver = true, this function should never be called.
-      should.ok(false, 'A content dom observer must not call the _ignoreTargetChange method.');
-      return true;
-    },
-  }
-);
+    {
+      _styleChangingAttributes: attrs,
+      _attributes: attrs,
+      _eventContentChange: eventContentChange,
+      _nestedTargetSelector: hostSelector,
+      _ignoreContentChange: (mutation, isNestedTarget) => {
+        const { target, attributeName } = mutation;
+        return isNestedTarget ? false : attributeName ? liesBetween(target as Element, hostSelector, '.content') : false;
+      },
+      _ignoreNestedTargetChange: (target, attrName, oldValue, newValue) => {
+        if (attrName === 'class' && oldValue && newValue) {
+          const diff = diffClass(oldValue, newValue);
+          const ignore = diff.length === 1 && diff[0].startsWith(ignorePrefix);
+          return ignore;
+        }
+        return false;
+      },
+      // @ts-ignore
+      _ignoreTargetChange: () => {
+        // if param: isContentObserver = true, this function should never be called.
+        should.ok(false, 'A content dom observer must not call the _ignoreTargetChange method.');
+        return true;
+      },
+    }
+  );
+};
+
+let contentDomObserver = createContentDomOserver(contentChangeArr);
 
 const getTotalObservations = () => domTargetObserverObservations.length + domContentObserverObservations.length;
 const getLast = <T>(arr: T[], indexFromLast = 0): T => arr[arr.length - 1 - indexFromLast] || ({} as T);
@@ -277,7 +272,7 @@ const addRemoveElementsTest = async (slot: Element | null, changeThrough?: DOMCo
       if (addChangeThrough) {
         const contentChanged = getLast(addChangeThrough);
         await waitForOrFailTest(() => {
-          should.equal(contentChanged, true, 'Adding an content element must result in a content change.');
+          should.deepEqual(contentChanged, { contentChange: true, troughEvent: false }, 'Adding an content element must result in a content change.');
         });
       }
     };
@@ -297,7 +292,11 @@ const addRemoveElementsTest = async (slot: Element | null, changeThrough?: DOMCo
 
           if (removeChangeThrough) {
             const contentChanged = getLast(removeChangeThrough);
-            should.equal(contentChanged, true, 'Removing an content element must result in a content change.');
+            should.deepEqual(
+              contentChanged,
+              { contentChange: true, troughEvent: false },
+              'Removing an content element must result in a content change.'
+            );
           }
         });
       }
@@ -361,10 +360,14 @@ const addRemoveImgElmsFn = async () => {
       compare(2);
 
       const previousContentChanged = getLast(domContentObserverObservations, 1);
-      should.equal(previousContentChanged, true, 'Adding an content image must result in a content change.');
+      should.deepEqual(
+        previousContentChanged,
+        { contentChange: true, troughEvent: false },
+        'Adding an content image must result in a content change.'
+      );
 
       const lastContentChanged = getLast(domContentObserverObservations);
-      should.equal(lastContentChanged, true, 'The images load event must result in a content change.');
+      should.deepEqual(lastContentChanged, { contentChange: true, troughEvent: true }, 'The images load event must result in a content change.');
     });
   };
 
@@ -375,21 +378,20 @@ const addRemoveImgElmsFn = async () => {
   // test event content change debounce
   const addMultiple = async () => {
     const { before, after, compare } = changedThrough(domContentObserverObservations);
-    const addMultipleItem = () => {
+    const genImage = () => {
       const img = new Image(1, 1);
       img.src = 'data:image/gif;base64,R0lGODlhAQABAAD/ACwAAAAAAQABAAACADs=';
 
       const imgHolder = createDiv('img');
       appendChildren(imgHolder, img);
 
-      appendChildren(imgElmsSlot, imgHolder);
+      return imgHolder;
     };
 
-    before();
+    await timeout(250);
 
-    addMultipleItem();
-    addMultipleItem();
-    addMultipleItem();
+    before();
+    appendChildren(imgElmsSlot, [genImage(), genImage(), genImage()]);
 
     await timeout(250);
 
@@ -398,20 +400,27 @@ const addRemoveImgElmsFn = async () => {
       compare(2);
 
       const previousContentChanged = getLast(domContentObserverObservations, 1);
-      should.equal(previousContentChanged, true, 'Adding mutliple content images must result in a single content change. (debounced)');
+      should.deepEqual(
+        previousContentChanged,
+        { contentChange: true, troughEvent: false },
+        'Adding mutliple content images must result in a single content change. (debounced)'
+      );
 
       const lastContentChanged = getLast(domContentObserverObservations);
-      should.equal(lastContentChanged, true, 'Multiple images load events must result in a single cintent change. (debounced)');
+      should.deepEqual(
+        lastContentChanged,
+        { contentChange: true, troughEvent: true },
+        'Multiple images load events must result in a single cintent change. (debounced)'
+      );
     });
   };
 
   await addMultiple();
 
   // remove load event from image test
-  const addChanged = async (
-    newEventContentChange: Array<[string | null | undefined, (() => string | null | undefined) | string | null | undefined] | null | undefined>
-  ) => {
-    contentDomObserver._updateEventContentChange(newEventContentChange);
+  const addChanged = async (newEventContentChange: Array<[string | null | undefined, string | null | undefined] | null | undefined>) => {
+    contentDomObserver._destroy();
+    contentDomObserver = createContentDomOserver(newEventContentChange);
 
     const img = new Image(1, 1);
     img.src = 'data:image/gif;base64,R0lGODlhAQABAAD/ACwAAAAAAQABAAACADs=';
@@ -430,7 +439,8 @@ const addRemoveImgElmsFn = async () => {
       compare(1);
     });
 
-    contentDomObserver._updateEventContentChange(contentChangeArr);
+    contentDomObserver._destroy();
+    contentDomObserver = createContentDomOserver(contentChangeArr);
   };
 
   await addChanged([
@@ -440,9 +450,6 @@ const addRemoveImgElmsFn = async () => {
     ['img', undefined],
     [null, null],
     [undefined, undefined],
-    ['img', () => 'hi'],
-    ['img', () => null],
-    ['img', () => undefined],
     null,
     undefined,
   ]);
@@ -471,7 +478,11 @@ const addRemoveTransitionElmsFn = async () => {
             compareTransition(expectTransitionEndContentChange ? 2 : 1); // 2 because 1: added class mutation and 2: transition end event
 
             const contentChanged = getLast(domContentObserverObservations);
-            should.equal(contentChanged, true, 'The transitionend event must trigger a event content change.');
+            should.deepEqual(
+              contentChanged,
+              { contentChange: true, troughEvent: expectTransitionEndContentChange },
+              'The transitionend event must trigger a event content change.'
+            );
             resolve(1);
           });
         },
@@ -495,11 +506,16 @@ const addRemoveTransitionElmsFn = async () => {
       compare(1);
 
       const contentChanged = getLast(domContentObserverObservations);
-      should.equal(contentChanged, true, 'Adding an content element (transition) must result in a content change.');
+      should.deepEqual(
+        contentChanged,
+        { contentChange: true, troughEvent: false },
+        'Adding an content element (transition) must result in a content change.'
+      );
     });
 
     await startTransition(elm, expectTransitionEndContentChange && true);
-    contentDomObserver._updateEventContentChange(contentChangeArr);
+    contentDomObserver._destroy();
+    contentDomObserver = createContentDomOserver(contentChangeArr);
     await startTransition(elm, expectTransitionEndContentChange && false);
 
     removeElements(elm);
@@ -509,19 +525,8 @@ const addRemoveTransitionElmsFn = async () => {
 
   await add(false);
 
-  contentDomObserver._updateEventContentChange(
-    contentChangeArr.concat([
-      [
-        '.transition',
-        (elms) => {
-          elms.forEach((elm) => {
-            should.equal(hasClass(elm as Element, 'transition'), true, 'Every checked element must match the correpsonding selector.'); // in this case "".transition"
-          });
-          return 'transitionend';
-        },
-      ],
-    ])
-  );
+  contentDomObserver._destroy();
+  contentDomObserver = createContentDomOserver(contentChangeArr.concat([['.transition', 'transitionend']]));
 
   await add(true);
 };
@@ -562,7 +567,11 @@ const iterateTargetAttrChange = async () => {
 const iterateContentAttrChange = async () => {
   await iterateAttrChange(setContentAttr, domContentObserverObservations, (observation) => {
     const contentChanged = observation;
-    should.equal(contentChanged, true, 'A attribute change inside the content must trigger a content change for a DOMContentObserver.');
+    should.deepEqual(
+      contentChanged,
+      { contentChange: true, troughEvent: false },
+      'A attribute change inside the content must trigger a content change for a DOMContentObserver.'
+    );
   });
   await iterateAttrChange(setFilteredContentAttr);
 };
@@ -573,7 +582,11 @@ const iterateContentBetweenAttrChange = async () => {
 const iterateContentHostElmAttrChange = async () => {
   await iterateAttrChange(setContentHostElmAttr, domContentObserverObservations, (observation) => {
     const contentChanged = observation;
-    should.equal(contentChanged, true, 'A attribute change for a nested target must trigger a content change for a DOMContentObserver.');
+    should.deepEqual(
+      contentChanged,
+      { contentChange: true, troughEvent: false },
+      'A attribute change for a nested target must trigger a content change for a DOMContentObserver.'
+    );
   });
   await iterateAttrChange(setFilteredContentHostElmAttr);
 };
@@ -626,11 +639,9 @@ const start = async () => {
   targetDomObserver._destroy();
   targetDomObserver._update();
 
-  contentDomObserver._updateEventContentChange([]);
   contentDomObserver._update();
   contentDomObserver._destroy();
   contentDomObserver._destroy();
-  contentDomObserver._updateEventContentChange([]);
   contentDomObserver._update();
 };
 
diff --git a/packages/overlayscrollbars/tests/browser/observers/sizeObserver/index.browser.ts b/packages/overlayscrollbars/tests/browser/observers/sizeObserver/index.browser.ts
index b9e0415..74b33c5 100644
--- a/packages/overlayscrollbars/tests/browser/observers/sizeObserver/index.browser.ts
+++ b/packages/overlayscrollbars/tests/browser/observers/sizeObserver/index.browser.ts
@@ -37,12 +37,14 @@ const preInitChildren = targetElm?.children.length;
 
 const sizeObserver = createSizeObserver(
   targetElm as HTMLElement,
-  (directionIsRTLCache?: any) => {
-    if (directionIsRTLCache) {
-      directionIterations += 1;
-    } else {
+  ({ _directionIsRTLCache, _sizeChanged, _appear }) => {
+    if (_sizeChanged) {
       sizeIterations += 1;
     }
+
+    if (_directionIsRTLCache) {
+      directionIterations += 1;
+    }
     requestAnimationFrame(() => {
       if (resizesSlot) {
         resizesSlot.textContent = (directionIterations + sizeIterations).toString();
diff --git a/rollup.config.base.js b/rollup.config.base.js
index d9e7af3..c8a8e44 100644
--- a/rollup.config.base.js
+++ b/rollup.config.base.js
@@ -268,7 +268,7 @@ const rollupConfig = (config = {}, { project = process.cwd(), overwrite = {}, si
                 plugins: [
                   ...(output.plugins || []),
                   rollupTerser({
-                    ecma: 8,
+                    ecma: esm ? 2015 : 5,
                     safari10: true,
                     mangle: {
                       safari10: true,