I see what you mean in that screencast, yeah. But really I think that video might be showing the difference between "overlay-style" scrollbars vs. traditional scrollbars. (And overlay-style scrollbars do indeed seem to create a more confusing experience here.) On your Windows 11 machine, could you try changing the system setting to activate "Always show scrollbars" and see if you still see a Firefox/Chrome difference at that point? (I think you can find that in Win11 settings, Accessibility > Visual Effects > Always show scrollbars.) (As you observed in comment 6, we don't have this available in Firefox's own settings on Win11 - we defer to the system setting.)
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I see what you mean in that screencast, yeah. But really I think that video might be showing the difference between "overlay-style" scrollbars vs. traditional scrollbars. (And overlay-style scrollbars do indeed seem to create a more confusing experience here due to the many nested scrollframes.) On your Windows 11 machine, could you try changing the system setting to activate "Always show scrollbars" and see if you still see a Firefox/Chrome difference at that point? (I think you can find that in Win11 settings, Accessibility > Visual Effects > Always show scrollbars.) (As you observed in comment 6, we don't have this available in Firefox's own settings on Win11 - we defer to the system setting.)