[Webkit-unassigned] [Bug 230669] New: iOS15 is causing important performance issue due to reflow

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=230669

            Bug ID: 230669
           Summary: iOS15 is causing important performance issue due to
                    reflow
           Product: WebKit
           Version: Safari 14
          Hardware: iPhone / iPad
                OS: Other
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: mic.gallego at gmail.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Hi,

I have found a major problem in iOS15 that is impacting most slider libraries, for instance. I have found the issue to be due to how iOS15 now handles the toolbar.

Previously, if the toolbar at the bottom was closed, moving up from 1 or 2 pixel would not cause the bar to goes up. In the context of a slider library, this means that if you would swipe left or right, the window height would not change unless you would do a rather large vertical movement.

However on iOS15, when you scroll a bit diagonally (to swipe for instance), the bottom bar is moving up and down for every pixel of your diagonal movement.

Unfortunately, this cause the "resize" event to be called a very large number of times. Libraries like Flickity therefore re-render them whole carousel a huge number of times per swipe movement, which cause everything to be laggy (having the bottom bar going up and down while dragging slighly diagonally is also just annoying, from UX perspective).

I think that iOS should introduce a minimum number of scrolled pixel before initiating the collapse of the bar OR triggering the resize event only after the bar has been minimized or shown, but now many times in between as it does now.

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