A CSS animation (using transform with a transition) is defined on the container’s :focus-within state to “float” and shrink the <label> when its corresponding <input> gains focus. In a Qt-based browser:

• The focus state is correctly detected (the CSS rule appears in DevTools).

• The style is updated (you can see the new transform in the inspector).

• However, the animation never actually runs, so the label remains static.

Only after opening DevTools and making any edit to the CSS (which forces a style refresh) does the animation finally trigger. In mainstream browsers (Chrome, Firefox, Safari), the transform/animation applies immediately on first focus without any manual CSS re-application.

Steps to reproduce:

• Open the attached file `focus-within-bug.html` in a Qt-based browser.
• Click into the text input, observe the label remains in its original position; no animation fires.
• Open DevTools, modify any character in the <style> block, then close DevTools.
• Click the input again, observer the label animates as expected (shrinks and floats).

Environment:

• OS: Windows
• This bug could be reproduced with Example Qt Browser (Qt WebEngine 6.9.0)
• This bug could not be reproduced with both Chromium 130.0.6723.192 and 133.0.6943.141 (the base and security versions declared by Qt 6.9) on Windows