Autocomplete: Fix IE/Edge scrolling issues

IE11 and scrolling autocompletes didn't get along great; this should help fix
their relationship.

When you click on an autocomplete scrollbar in IE11, the menu temporarily
gains focus, which caused a couple problems.

1. Depending on how long you clicked, the dropdown could close.

2. Scrolling down by clicking the scrollbar's down arrow would misbehave. The
list would pop back up to the top with the first item selected.

We can fix both problems by modifying the focus/blur handling a bit.

1. There is a flag to instruct the control to ignore blurs, but it was getting
cleared too quickly; when the code refocused the input after it was blurred,
IE would send *another* blur event, which wasn't getting ignored and would
close the dropdown. We now wait for the focus/blur pair to process before
clearing the flag.

2. We remove the tabindex from the dropdown menu, which prevents menu's focus
handler from firing. When you focus a menu, it will select the first menu item
if none are selected. Selecting a menu item will scroll it into view if it's
not visible. This combination of behaviors was causing the strange behavior
when attempting to scroll down.

I couldn't figure out a way to write a unit test for this, since it's IE only
and seems to require user interaction. You can verify the previous behavior
(and the fix) on `demos/autocomplete/maxheight.html`

Fixes #9638
Closes gh-1785

ui/widgets/autocomplete.js

@@ -200,11 +200,6 @@ $.widget( "ui.autocomplete", {
 				this.previous = this._value();
 			},
 			blur: function( event ) {
-				if ( this.cancelBlur ) {
-					delete this.cancelBlur;
-					return;
-				}
-
 				clearTimeout( this.searching );
 				this.close( event );
 				this._change( event );
@@ -220,31 +215,24 @@ $.widget( "ui.autocomplete", {
 				role: null
 			} )
 			.hide()
+
+			// Support: IE 11 only, Edge <= 14
+			// For other browsers, we preventDefault() on the mousedown event
+			// to keep the dropdown from taking focus from the input. This doesn't
+			// work for IE/Edge, causing problems with selection and scrolling (#9638)
+			// Happily, IE and Edge support an "unselectable" attribute that
+			// prevents an element from receiving focus, exactly what we want here.
+			.attr( {
+				"unselectable": "on"
+			} )
 			.menu( "instance" );
 
 		this._addClass( this.menu.element, "ui-autocomplete", "ui-front" );
 		this._on( this.menu.element, {
 			mousedown: function( event ) {
 
-				// prevent moving focus out of the text field
+				// Prevent moving focus out of the text field
 				event.preventDefault();
-
-				// IE doesn't prevent moving focus even with event.preventDefault()
-				// so we set a flag to know when we should ignore the blur event
-				this.cancelBlur = true;
-				this._delay( function() {
-					delete this.cancelBlur;
-
-					// Support: IE 8 only
-					// Right clicking a menu item or selecting text from the menu items will
-					// result in focus moving out of the input. However, we've already received
-					// and ignored the blur event because of the cancelBlur flag set above. So
-					// we restore focus to ensure that the menu closes properly based on the user's
-					// next actions.
-					if ( this.element[ 0 ] !== $.ui.safeActiveElement( this.document[ 0 ] ) ) {
-						this.element.trigger( "focus" );
-					}
-				} );
 			},
 			menufocus: function( event, ui ) {
 				var label, item;