[Webkit-unassigned] [Bug 177646] Scroll containers with "-webkit-overflow-scrolling: touch" don't always show scrollbars on scroll

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=177646

tahoon at microsoft.com changed:

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                 CC|                            |tahoon at microsoft.com

--- Comment #4 from tahoon at microsoft.com ---
Created attachment 361714

  --> https://bugs.webkit.org/attachment.cgi?id=361714&action=review

Repro case: rapid changes to scrollable height

This bug is timing-dependent. The symptom is that the user can swipe up and down to scroll an element; however, the scrollbar does not appear. Workaround is to change "-webkit-overflow-scrolling" to "auto" and then back to "touch" after a delay.

Attachment provides two ways to reproduce this bug in iOS.
The first way is reliable:
1. Check the "Add items automatically" checkbox. This adds items continuously to a scrollable area.
2. Flick finger up and down to scroll the list. Note that the scrollbar disappears (or does not appear in the first place).
3. Uncheck checkbox. This stops adding items.
4. Flick finger up and down. The list scrolls but there is no scrollbar.

The second way is to add items manually. It is more difficult to reproduce.
1. Click "Add item" button quickly 4 to 5 times.
2. Lightly and quickly flick finger up on the scrollable area.
3. Area scrolls but scrollbar does not appear.
4. If cannot reproduce, click "Clear all" and try again.

I have also managed to reproduce this bug with static HTML (no dynamic DOM modifications), but very rarely.

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