At certain screen sizes Firefox "flickers" between sizes as scrollbars flash on and off

[stuartlangridge]

Resize Measure horizontally in a Firefox window. At certain screen widths, the scrollbars on widgets start flickering on and off. I think the scrollbars turn on, which fractionally changes the size of everything, which means that the widgets are now big enough to not need scrollbars, which means they turn off, which means everything changes size a little bit, and then we go back to the beginning again; hence, flicker.

Doesn't seem to happen in Chrome. Could be a grid layout issue, or maybe we need to be cleverer about scrollbar vertical overflow.