scrollbarSize() calculation issue for scaled page (FireFox)

During the scroll bar size calculation the test div element with 50x50px is temporary created. But from some page scale (50%) this isn't enough for scrollbars (or scroll buttons, I'm not sure) and this function returns size 0.
Suppose that larger div should used (in my case it was 200x200).

Start by locating the scrollbarSize() function and its temporary 50x50px test div. Reproduce the calculation in Firefox at 50% page scale, then verify that using a larger test element prevents a zero scrollbar size and preserves the expected result at normal scale.

Written by the indexing model from the issue text.