I see the size change on this bugzilla page on macOS. During scrolling the scrollbar has size A, and then once it starts fading out, it jumps to a slightly bigger size B. So if you're towards the top of the document, you see the scrollbar's bottom end grow downwards; if you're towards the bottom, you see the upper end grow upwards; and if you're in the middle of the document, you see both ends grow outwards. Actually, experimenting more, it seems that the scrollbar is small whenever the scroll offset hasn't changed since the previous drawn frame and large whenever the scroll offset *has* changed since the previous frame. I can see the the large size whenever a new main thread paint happens, for example if I slowly scroll over a link with hover feedback.
Bug 1818702 Comment 24 Edit History
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I see the size change on this bugzilla page on macOS when scrolling with the touchpad. During scrolling the scrollbar has size A, and then once it starts fading out, it jumps to a slightly bigger size B. So if you're towards the top of the document, you see the scrollbar's bottom end grow downwards; if you're towards the bottom, you see the upper end grow upwards; and if you're in the middle of the document, you see both ends grow outwards. Actually, experimenting more, it seems that the scrollbar is small whenever the scroll offset hasn't changed since the previous drawn frame and large whenever the scroll offset *has* changed since the previous frame. I can see the the large size whenever a new main thread paint happens, for example if I slowly scroll over a link with hover feedback.
I see the scrollbar size change on this bugzilla page on macOS when scrolling with the touchpad. During scrolling the scrollbar has size A, and then once it starts fading out, it jumps to a slightly bigger size B. So if you're towards the top of the document, you see the scrollbar's bottom end grow downwards; if you're towards the bottom, you see the upper end grow upwards; and if you're in the middle of the document, you see both ends grow outwards. Actually, experimenting more, it seems that the scrollbar is small whenever the scroll offset hasn't changed since the previous drawn frame and large whenever the scroll offset *has* changed since the previous frame. I can see the the large size whenever a new main thread paint happens, for example if I slowly scroll over a link with hover feedback.