[Webkit-unassigned] [Bug 226764] New: Complex flex layout is very slow to load

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=226764

            Bug ID: 226764
           Summary: Complex flex layout is very slow to load
           Product: WebKit
           Version: Safari 14
          Hardware: iPhone / iPad
                OS: iOS 14
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: pguz at datapa.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Our dashboards are built using 'splitter' components that use flex.  A splitter may be either in a horizontal (flex:row) or vertical (flex:column) orientation.  Splitters are nested, potentially deeply, to divide up the dashboard.

We have found complex dashboards extremely slow to load on iPads.  Testing on Safari on an iPad Air 2 (MGLW2ZP/A) running iPadOS 14.6, the following fiddle takes 35 seconds to load. http://bitly.ws/e45Y

Compare with the following fiddle, with flex removed: http://bitly.ws/e45Y

The same is observed in Chrome on the same iPad.

Desktop browsers and Android tablets of a similar specification do not exhibit this slow performance.

I do not have easy access to MacOS or Linux at this time so cannot compare with desktop WebKit, nor can I test on Technology Preview.

A similar bug (https://bugs.webkit.org/show_bug.cgi?id=222202) was recently fixed, and I believe that fix would have been shipped with iPadOS 14.5.  Similarly to this bug, we use % widths.  However, unlike the test attached to that bug, we nest both column and row layouts.

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