Fixes #11610: Add @grid-float-breakpoint-max to better link navbar behavior across viewports and improve customization.

Also addresses #10371, #10395, and #10465.

less/variables.less

@@ -247,8 +247,13 @@
 @grid-columns:              12;
 // Padding, to be divided by two and applied to the left and right of all columns
 @grid-gutter-width:         30px;
-// Point at which the navbar stops collapsing
+
+// Navbar collapse
+
+// Point at which the navbar becomes uncollapsed
 @grid-float-breakpoint:     @screen-sm-min;
+// Point at which the navbar begins collapsing
+@grid-float-breakpoint-max: (@grid-float-breakpoint - 1);
 
 
 // Navbar