moving upstream to OOUI

We probably just need the translate:transformZ(0) hack (applied to oo-ui-window-body) to trigger hardware acceleration, but we need to consider the z-index implications.

Actually it would need to be applied to all scrollable panels.

Change 330068 had a related patch set uploaded (by Esanders):
Apply webkit GPU hack to scrollable panels

https://gerrit.wikimedia.org/r/330068

Interesting read about this: http://stackoverflow.com/questions/39261456/reducing-scroll-jank-when-using-overflow-y-scroll/39377543#39377543

Change 330068 merged by jenkins-bot:
Apply webkit GPU hack to scrollable panels

https://gerrit.wikimedia.org/r/330068

This is such a undocumented mess on browser level. I've been revisiting the GPU hack documentation and articles for several hours the x-th time and only stumble to a clear understanding of what code is needed and appropriate. Filed T154460: Revisit OOUI's GPU hack mixin `transform: translate3d()` as follow-up.

I've got the impression that a slight repositioning render issue on Chrome is connected with the current, as of v0.24.3, .oo-ui-force-gpu-composite-layer() hack on scrollable PanelLayout. It's pretty hard to reproduce, but it affects for example save dialog or the Page Settings dialog in VE which make use of scrollable PanelLayout. When you hover over certain elements like the checkboxes in the dialog, they move slightly (~by 1 px). This hasn't been always the case and the hack seems like a good candidate for this issue.

Another instance, I can now repeatedly pin-point it to this hack: