Programmatically detect when mouse approaches the edge of a dock

Previously, we would assign dock elements a minimum width/height of 2
pixels so that we could detect when the mouse approached the edge of a
hidden dock in order to show the chevron buttons. This, however, was
causing confusion for users, who expected that extra space to be
clickable in order to scroll editors located in the center dock.

With this commit we will instead register a global `mousemove` event on
the window right when attaching the workspace element to the DOM (the
event handler is debounced, so this shouldn't have any performance
consequence). Then, when mouse moves, we will programmatically detect
when it is approaching to the edge of a dock and show the chevron button
accordingly. This allows us to remove the `min-width` property from the
dock container element, which eliminates the confusing behavior
described above.

src/dock.js

@@ -327,12 +327,15 @@ module.exports = class Dock {
     // Include all panels that are closer to the edge than the dock in our calculations.
     switch (this.location) {
       case 'right':
+        if (!this.isVisible()) bounds.left = bounds.right - 2
         bounds.right = Number.POSITIVE_INFINITY
         break
       case 'bottom':
+        if (!this.isVisible()) bounds.top = bounds.bottom - 1
         bounds.bottom = Number.POSITIVE_INFINITY
         break
       case 'left':
+        if (!this.isVisible()) bounds.right = bounds.left + 2
         bounds.left = Number.NEGATIVE_INFINITY
         break
     }