SUMMARY
Changing the window title font size in the fonts KCM does seem to affect the title bar buttons size, which comes off as an unwanted side effect as title bar buttons already have a dedicated setting to change their size in the window decoration config window.

STEPS TO REPRODUCE
1. Open kcm_fonts
2. Change window title font size
3. Apply

OBSERVED RESULT
If the font size was increased, the whole title bar gets bigger, which is expected to fit the bigger text, but the buttons also get bigger which may or may not be wanted and may conflict with the dedicated setting to change their size in the decoration config.

Decreasing the font size poses a bigger problem because it makes the whole title bar and its buttons shrink down to potentially unusable sizes (https://bugs.kde.org/show_bug.cgi?id=505175).

One would expect a font setting to only change text not non-text controls like icons and buttons so why does this happen?

EXPECTED RESULT
Title bar buttons should only follow the dedicated sizing option in the window decoration settings, and the title bar should get its height from the taller of either the title text or the buttons so that both fit.

SOFTWARE/OS VERSIONS
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.14.0
Qt Version: 6.9.0