Scaling issues

[DavideRossi]

I have a scaling problem with Mint 19 Mate edition.
My laptop's screen has a 3200*1800 native resolution that I keep at 1600*900 since HiDPI is getting close to be OK but it's still not perfect.
The issue I'm having now, that wasn't there with Mint 18, is that several applications have scaling issues, appearing much bigger then they should.
This includes KeePassXC, TeXstudio, and a few others.
I tried poking around with the HiDPI settings in MATE Tweak with no success.
It seems like the system stored somewhere a DPI settings (that is used only by some applications) that is OK for the native resolution (3200*1800) but that is too high for the resolution I use (which compromises also the visualization on my external 2560*1440 external monitor).

Suggestions?

Thanks.

[Chrisj01]

Sorry if you have already done this, but, with your scaling set to 2x, have you tried going to "Appearance" and the "Fonst" tab, then click on "details" and then reducing the text DPI?

[DavideRossi]

The resolution is set to "Automatic detection" which turns out to be 96 DPI. That's OK for most programs. The vast majority of the programs are scaled correctly, but some seem to read a weird DPI number that must be hidden somewhere.
I bet this value has been set up during the installation, when the display was set to native 3200*1800...
If I only could figure out what smplayer, TeXstudio and KeePassXC have in common...

[DavideRossi]

This thing is really annoying: smplayer and vlc are both affected.
Can I ask somebody using a hidpi screen to try to set a lower resolution and check if they have scaling issues with the aforementioned programs?

[thx-1138]

...................
If I only could figure out what smplayer, TeXstudio and KeePassXC have in common...

https://wiki.archlinux.org/index.php/HiDPI#Qt_5

Step-by-step instructions here as well.

[DavideRossi]

Yes, it was QT. Getting closer but still not right.
Something is setting QT_SCALE_FACTOR back to "2" AFTER I set it to "1" in .profile.
I'm grepping around to see what does it but until now no success...

[thx-1138]

...not on 19, so I can't check directly. Note you might need to log-off (...or reboot) first for such to take permanent effect.
But if afterwards something still 'resets' it, then i'd look under /etc/profile.d/ and/or /etc/environment
(obviously be extra careful modifying system-wide files)...

[DavideRossi]

I even tried using .xsessionrc (the last time I wrote something there it was back in the 90s) but still the setting is overwritten.
I found nothing relevant in /etc, but I found that disabling "MATE Settings Daemon" from Control Center/Startup Applications solves the issue... but also does not set the resolution to 1600*900 so it keeps 3200*1800 without scaling.
Then I went into the code and I found a scale_change_workarounds function (telling name...) in the xsettings plugin (file msd-xsettings-manager.c) that deals with various QT settings. I see it first reads from the gsettings.. I'll try to play with these...

[DavideRossi]

That was indeed the case. I had to set org.mate.desktop.interface.window-scaling-factor to 1 to avoid the call to get_window_scale_auto in msd-xsettings-manager.c. But that also means that I'm basically breaking HiDPI support. I opened a bug report in github, I will report how this develops.