Hi Ioan,
As for numeric up/down and slider, color handling is not easy to solve, because there are images involved (fixed colors) and a lot of objects and related colors (standard, hovered, selected, etc...)
Width/height are driven by the content (but you are right, there's a bug in the preview changing the font settings - thank you [fixed])
DoubleClick is enabled in the header of the region and in the selected item (that shown in the header) of the drow-down list - and only if the worksheet is not under evaluation and - yes - is a little bit faster than the standard (this because the menu collapse/extension is driven by the double-click event listener, so to reduce the lag I've reduced the DoubleClick timing - from 300ms to 250ms - but I can restore the standard [done]).
> The liste closing when lost focus event seems deleyed sometimes (?). This could be also linked to the system.
I've made some improvements in the 0.0.5229.36040 version, while the 0.0.5229.28411 shows a white list when you select an item on slow systems/heavy worksheets (don't know what vrsion have you tested). Don't know if this is the issue that you have noticed (for loosing focus I consider that you click in a zone of the canvas outside the region or you press the tab key - this is really fast on my system, even on heavy worksheets)