I've just updated Arch Linux with KDE to Plasma 5.12.5 (I forgot to check which version I had before, but the last update I did was about a month ago) and I noticed the fonts in GTK applications are now ever so slighty larger than in QT applications.
I'm using an ultrawide 4K display, but I don't use scaling/HiDPI, so everything's set at 96 DPI and scale factor 1. The KDE font setting is set to 'Force DPI to: 96 DPI' and
gtk-query-settings gives me
gtk-xft-dpi: 98304, which is also 96 DPI.
When I set the KDE font setting to force DPI to 90, the GTK applications do look OK, however with fuzzy icons, and of course the QT applications look too small. But this does point to DPI being the culprit.
export GDK_SCALE=1 export GDK_DPI_SCALE=0.5
...to see whether this had any effect, but this does not make a difference.
Before I updated the system, everything was fine, just performing the update made the change.
I'm using a Nvidia card, which might have something to do with it, however the driver looks good and so does
nvidia-setings, which properly lists resolution and DPI.
Furthermore, I'm using default Breeze theme, nothing fancy, with Noto Sans font. The difference in font size can already be seen in the system settings GTK preview window (left is Noto Sans 10, right should be as well, but is obviously larger):
Any ideas what may be happening here?
It seems the problem is with the font I'm using, which is the default Noto Sans. The new version of this font is not correct or does not render correctly. The same issue is described here: https://github.com/googlei18n/noto-fonts/issues/1139
@jasonwryan deemed it necessary to remove the 'arch-linux' tag I included, but I'm adding the tag again, because it might very well be a packaging/distro specific issue (as mentioned in the GitHub post).
Since the issue on GitHub remains open, this question is not resolved yet either.
There seems to have been an update of the
noto-fonts package. Current version is
noto-fonts-20181024-1. The font still does not look exactly the same as in KDE applications, but the difference is less obvious.