Installed 10.3.0.5771 today — retina fonts broke (look pixelated).
Previously was using some 10.2.x version.
Installed HeidiSQL_10.2.0.5725_Setup.exe — fonts aren't rendered correctly.
Installed HeidiSQL_10.2.0.5599_Setup.exe — it renders fonts correctly.
Fonts are pixelated.
Fonts being smooth.
Use HeidiSQL_10.2.0.5599_Setup.exe.
https://www.heidisql.com/installers/HeidiSQL_10.2.0.5599_Setup.exe
Windows 10, 4k screen at 200% scaling.
This is due to the given up high DPI awareness. There were numerous bugs left in high DPI mode, which I could not solve, so I finally reverted the code for it.
I think you can still activate application based scaling, when you rightclick your taskbar icon for HeidiSQL, then click "Properties", then click "Compatibility" tab, then click "High DPI settings":

@ansgarbecker Ok, thx. 10.2 is fine, it'll be enough for 202x.
Note that the above overriding should also work on HeidiSQL v10.3. HeidiSQL should look sharp on scaled screens then, but this will include the same bugs as v10.2 also had.
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Note that the above overriding should also work on HeidiSQL v10.3. HeidiSQL should look sharp on scaled screens then, but this will include the same bugs as v10.2 also had.