fix(ui-select): avoid dropdown flickering on search

[m3co-code]

I hope commit message and subject are descriptive enough :) This is a fix for #1298 with no other side effects then the flickering.

[wesleycho]

This LGTM

[user378230]

Removing this introduces a problem when dropdown is positioned up

http://plnkr.co/edit/rM3rdaekwqG8wN77Hgb6?p=preview

Enter text in select2 "up" and delete with backspace quickly (I typed ada to get most results).

With this line removed the select gets shifted/overlayed with the list as a flicker. (Comment/uncomment line ~1261)

We need to detect if dropdown is positioned up and apply this along with your additions on line 298.

[m3co-code]

Thanks a lot for feedback! I will have a look tomorrow.

[m3co-code]

I had a look again in this issue. Sorry for the time delay guys..

I see the point of @user378230, when you reduce your search and thus extend the list again, the dropdown flickers for a short moment below the placeholder and then positions up. Though I can not figure out what is actually causing this or how to detect it..

I set up a plunkr with a couple of log messages where I tried to figure this out. Maybe you can give some useful input where to continue the investigation.

Please have a look in this Plunkr on lines 1259 and following. http://plnkr.co/edit/7NGlK9SRi95jNYTqpX1D?p=preview

[user378230]

@marco-jantke a shot in the dark but I wonder if the changes in #1123 help at all?

edit: I tested in the plunkr and it helps but I wonder maybe your PR isn't required if #1123 is merged instead...?

For reference I commented these:

var setDropdownPosUp = function(offset, offsetDropdown){

          offset = offset || uisOffset(element);
          offsetDropdown = offsetDropdown || uisOffset(dropdown);

          //dropdown[0].style.position = 'absolute';
          //dropdown[0].style.top = (offsetDropdown.height * -1) + 'px';
          element.addClass(directionUpClassName);

        };

        var setDropdownPosDown = function(offset, offsetDropdown){

          element.removeClass(directionUpClassName);

          offset = offset || uisOffset(element);
          offsetDropdown = offsetDropdown || uisOffset(dropdown);

          //dropdown[0].style.position = '';
          //dropdown[0].style.top = '';

        };

Because of changes implemented since #1123 was made

[rpocklin]

This fixes the flicker on search, but there was a reason this was added in the first place
// Hide the dropdown so there is no flicker until $timeout is done executing.

The goal should be to avoid the opacity reset if the dropdown is already visible, i've created a tweaked PR in #1594