ComboBox: drop down alignment off due to scrollbar (RTL)

[bill]

See test_ComboBox.html. On FF and IE, the third drop down is misaligned by the width of the scroll bar:

Also, on IE, after removing the DOCTYPE declaration from _autoComplete.html (forcing it to quirks mode), the second drop down box is slightly misaligned:

I assume this is different than #11376 since it happens when the document isn't scrolled horizontally.

There's also a possibly related problem where click the drop down button makes the document jump back to having no horizontal scroll.

[bill]

I suspect the problem is due to not accounting for the scrollbar (and which side of the drop down it appears on).

[bill]

I'll fix this as part of #9945

[bill]

(In [22660]) Convert ComboBox to extend _HasDropDown. The migration involved:

• make ComboBox use the code in _HasDropDown to open/close/size the drop down, and the code to monitor for clicks/keypresses to open/close drop down
• a lot of renaming: this._popupWidget --> this.dropDown, this.downArrowNode --> this._buttonNode, this.comboNode --> this.domNode, this._isShowingNow --> this._opened, _onKeyPress --> _onKey, _hideResultList --> closeDropDown
• removed "state" from _HasDropDown since it conflicted with ValidationTextBox.state and since it had the same meaning as this._opened
• added maxHeight: -1 setting to _HasDropDown to cap the drop down size to what will fit in the viewport

Since ComboBox's drop down opens asynchronously this was a tricky change, because _HasDropDown wasn't really designed for the drop down to open asynchronously, except for the first time it's shown, when it's loading from an href.

Fixes #9945, #11461 !strict. Refs #10229 but doesn't fix completely.