[Webkit-unassigned] [Bug 158342] New: Overlay with -webkit-overflow-scrolling:touch doesn't become scrollable after added text makes it taller

Fri Jun 3 00:04:38 PDT 2016

https://bugs.webkit.org/show_bug.cgi?id=158342

            Bug ID: 158342
           Summary: Overlay with -webkit-overflow-scrolling:touch doesn't
                    become scrollable after added text makes it taller
    Classification: Unclassified
           Product: WebKit
           Version: Safari 9
          Hardware: iOS
                OS: iOS 9.3
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: webkit at chrisrebert.com
                CC: simon.fraser at apple.com

Steps to reproduce:
1. Open http://output.jsbin.com/tuxahu/quiet in iOS 9.3 Safari.
2. Tap the "Go" button.
3. Observe that a modal dialog appears with text indicating that its content is loading.
4. Wait for the "loading" text to go away and be replaced with lot of other text,
     causing the modal to become taller than the screen.
5. Attempt to scroll the modal downward to read the rest of the text.

Actual result:
The underlying <body> below the modal scrolls temporarily (despite being overflow:hidden),
and the modal within the topmost position:fixed "layer" doesn't move at all.

Expected result:
The modal should move upward, revealing more of the text.

Original Bootstrap issue:
https://github.com/twbs/bootstrap/issues/17695

Further details:
This sounds similar to https://bugs.webkit.org/show_bug.cgi?id=150974
The bug seems to be related to `-webkit-overflow-scrolling:touch`.
Setting `-webkit-overflow-scrolling:auto` on the `.modal` fixes the problem.
I also observed that merely toggling the `.modal-open .modal { overflow-y: auto; }` style
off and back on again in Safari DevTools (while connected to the Simulator,
and leaving `-webkit-overflow-scrolling:touch` intact) is sufficient to cause the modal to become scrollable again.

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