Different browsers have differing implementations of the .scrollLeft property for RTL content (nicely described at <https://github.com/othree/jquery.rtl-scroll-type#3-types-of-scrollleft-scrollwidth--100>) and it looks like we never handle these differences in OOjs UI, so presumably almost all of our usages of .scrollLeft result in incorrect behavior in RTL. We should look at them and fix them.
https://gerrit.wikimedia.org/r/339053 adds OO.ui.Element.static.getScrollLeft(), which is a consistent getter method for this. There's no setter method currently.
Currently, the following functions use scrollLeft:
** Looks OK, we don't look at the value, only get it and set it back.
** This is actually `$(window).scrollLeft()`, which is another can of worms.