https://bugreports.qt.io/browse/QTBUG-66036 was a visual issue affecting Kate, KWrite, and KDevelop when run using a non-integer scale factor. It's been fixed, but the fix only landed in Qt 5.12, which hasn't even been released yet. To improve the HiDPI experience for our users, we should consider cherry-picking the fix into our Qt 5.10 and 5.11 branches (not into 5.9 for the LTS though).
we should consider cherry-picking the fix into our Qt 5.10 and 5.11 branches
5.11 is going into Cosmic now, and into Debian right after the ffmpeg transition is done. I've uploaded the fix to Cosmic (it's in -proposed right now while the transition is happening) and Debian Experimental (staging area before Sid).
Where do we have 5.10 besides KCI?
not into 5.9 for the LTS though
Is this not a problem with 5.9?
KCI and other experimental PPAs is the only place we had 5.10, and then only as then only as the 5.11 debian packaging for 5.11 did not exist at the time. All that can update to 5.11 when it's ready.