The way to configure HiDPI font and icon scaling in KDE has changed at least a couple of
times: (Note: Some programs, such as Chromium and Java, ignore all these settings and
have to be configured independently.)
1. In the original implementation of HiDPI support for KDE, as documented in blog posts by
KDE developer David Edmundson, it was necessary to set both "force font DPI" -
for fonts - and "scale display" - for everything else.
2. In January when I first installed Fedora 23, setting both "force font DPI"
and "scale display" produced unexpected results - I eventually figured out that
in fact, it worked much better if I just set "scale display" to 2.
3. I've since changed jobs and installed Fedora 23 on a new laptop, and with all the
updates installed, it's again necessary to set both "force font DPI" and
"scale display" for everything to be legible without squinting. But this
it's undesirable to set both, because now with both set, most KDE applications have
huge fonts. Non-KDE applications like Thunderbird look OK though.
Some possible clues as to why this has changed again:
Clue 1: If I leave "force font DPI" unset, it's actually set to 0 in a
config file, and the fonts are really tiny, suggesting that 0 is being interpreted as
Clue 2: I now get these messages when I run a KDE application from a konsole:
Warning: QT_DEVICE_PIXEL_RATIO is deprecated. Instead use:
QT_AUTO_SCREEN_SCALE_FACTOR to enable platform plugin controlled per-screen factors.
QT_SCREEN_SCALE_FACTORS to set per-screen factors.
QT_SCALE_FACTOR to set the application global scale factor.
So I'm guessing that Qt has been upgraded to a newer version, but KDE hasn't been
updated to take account of the changes mentioned above.
Since they are environment variables, it looks like I can set them myself without needing
to wait for KDE to be fixed. So how should I set them?