[Webkit-unassigned] [Bug 173887] New: Scroll snap behaves unpredictably on body element, when combined with fixed overlay and changing content.

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=173887

            Bug ID: 173887
           Summary: Scroll snap behaves unpredictably on body element,
                    when combined with fixed overlay and changing content.
           Product: WebKit
           Version: Safari 10
          Hardware: iPhone / iPad
                OS: iOS 10.3
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: lists at scratchdisk.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

I am trying to get scroll-snap to work on the body element in iOS, so that the navigation bar is allowed to disappear when the user scrolls the website, all while supporting scroll-snapping to snap to full pages.

This works well as long as I don't add an overlay element with changing content that is always visible using `position: fixed;` styling.

Here a test-case that illustrates the faulty behavior on iOS:

https://safari-scroll-snap-bug.netlify.com

On Safari / macOS and on other browsers, this behaves as it should. On iOS, I see this faulty behavior that leads to an uncontrollable website and excessive flickering sometimes:

https://safari-scroll-snap-bug.netlify.com/video.mp4

It looks as if the changes in the `#overlay` element are triggering a reflow in the `body`, which then confused the scroll snap behavior. This is surprising, as it has `position: fixed;` set and changes in it should not affect the layout underneath.

Another possible factor could be the changing view height due to the disappearing navigation bars. 

But once I add the `display: none;` to the `#overlay` element, the scroll behaves as it should.

The only workaround I could find so far is setting `overflow: hidden` on `body` and having `.pages` scroll instead, but this in turn breaks the hiding of navigation bars on iOS, which is not desirable.

I am hopeful that this information will assist with a timely fix of this bug.

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