Incorrect sizing and bad text rendering with WebEngine using fractional scaling on Wayland

After Anki updated to Qt 6.5.0, Text rendered inside a webview looked worse and some content was too small unless the window was resized. This problem can be reproduced outside of Anki.

The sizing issue doesn't always occur; using the attached test script, I noticed that it only appeared if the webview was resized to a height of at least 596 px (on my 1080p display).

Since Qt 6.5.0 added support for the fractional_scale_v1 wayland extension, I guess that this problem might be caused by an incomplete implementation on the WebEngine side, although I have no deeper insight into this.

Original report: https://github.com/ankitects/anki/issues/1767#issuecomment-1535050698

Steps to reproduce:

•  Use the latest version of GNOME or KDE and enable fractional scaling
  • (only GNOME): 

    gsettings set org.gnome.mutter experimental-features "['scale-monitor-framebuffer']"
    

• • set your screen to 125% scaling in GNOME Settings/KDE System Settings
• run the attached script to render a web page

  python3 -m venv venv
  ./venv/bin/pip install pyside6
  ./venv/bin/python qt_webengine_fractional_scaling.py
  

If you need more information, I will be happy to provide it.