[Webkit-unassigned] [Bug 193198] New: Wrong root element dimensions when page with overflowing elements being scaled down in split screen

Mon Jan 7 08:07:24 PST 2019

https://bugs.webkit.org/show_bug.cgi?id=193198

            Bug ID: 193198
           Summary: Wrong root element dimensions when page with
                    overflowing elements being scaled down in split screen
           Product: WebKit
           Version: Safari 12
          Hardware: Macintosh
                OS: macOS 10.14
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: riku.mi at outlook.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 358497

  --> https://bugs.webkit.org/attachment.cgi?id=358497&action=review

Screenshot

Steps to reproduce:

1. Construct a page with at least one element overflowing viewport:
    ```
    <html><head><style>
    body {
        width: 100%;
        margin: 10px;
    }
    body::before {
        content: '';
        position: absolute;
        width: 100%;
        height: 100%;
        background: #777;
    }
    </style></head><body></body></html>
    ```
2. On Safari 12, go to Safari -> Preferences -> Websites -> Page Zoom, and scale the current page down (50% is best to reproduce the bug).
3. Set Safari to fullscreen.
4. Open the constructed page.
5. Split the screen with another window (or the devtools).
6. Drag the separator to give the Safari window enough width (>= 1100pt).
7. Drag the separator again to reduce the width (< 1100pt).
8. The page turns into bad layout, where the root element is half the width of viewport (when the zoom factor is 50%).

Since tons of public sites does not use `overflow:hidden` for `<html>`, this bug can be easily reproduced.

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