[Webkit-unassigned] [Bug 253399] New: Incorrect cropping of fixed position elements taller than viewport on MobileSafari

Sat Mar 4 14:47:45 PST 2023

https://bugs.webkit.org/show_bug.cgi?id=253399

            Bug ID: 253399
           Summary: Incorrect cropping of fixed position elements taller
                    than viewport on MobileSafari
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: iPhone / iPad
                OS: iOS 16
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: luke at deentaylor.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 465298

  --> https://bugs.webkit.org/attachment.cgi?id=465298&action=review

Showing the buggy behavior (first) and the desired behavior (second)

On iOS Safari, a fixed position element that is (1) taller than the viewport and (2) non-empty will be visually cropped to 100dvh height even if it is taller than that. This creates awkward jank when scrolling, because the element is visibly too small while the bottom UI is collapsing on scroll (see the first part of the attached video, with the red background). If the element is empty this bug does not occur (see the second part of the attached animation, with the green background—the element is visible behind the bottom bar and no visible jank occurs when the bottom bar collapses).

The incorrect behavior can be reproduced with the following reproduction:

<!DOCTYPE html>
<html lang="en">

<head>
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>

<body style="background: blue; margin: 0; padding-bottom: 300vh;">
  <div style="display: contents;">
    <div style="position: fixed; height: 100lvh; width: 100%; background: red;">
      test
    </div>
  </div>
</body>

</html>

The bug does not occur if the div is empty, i.e. if “test” is removed or commented out.

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