Incorrect UI scaling with high DPI laptop screen

[jbp35]

What is the bug or the crash?

The UI of QGIS is not scaled properly on Windows 10 when using a high DPI laptop screen with an additional external monitor.

Steps to reproduce the issue

Install and run QGIS with 4K high DPI screen.

Versions

All versions up to 3.26

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

The DevicePixelRatio of the 4K screen does not seems to be set correctly in QGIS as we can see in this comparison of QGIS console output and the standalone DprGadget provided by QT:

This appears to me that this is due to the fact that the flag AA_EnableHighDpiScaling is not set before the creation of the QApplication. This seems to be set by default in QT6 but not in QT5.

I am able to get everything working as expected by setting this manually and running QGIS in windows console:

set QT_ENABLE_HIGHDPI_SCALING=1
C:\OSGeo4W\bin\qgis-bin.exe

In addition, it is useful to also set :
QT_SCALE_FACTOR_ROUNDING_POLICY=passthrough
This is necessary as my windows scaling factor is not an integer (250%). Otherwise, the DevicePixelRatio for my High DPI screen is round up to 3 instead of 2.5 which makes the icons look a little too big. QT documentation mention that setting non integer scaling factor can cause issues with some styles, but in my case I have not seen any problem so far.

[nicogodet]

See #40274
Duplicates ?

[jbp35]

See #40274 Duplicates ?

I have indeed the same issue on my 4K laptop screen. However, #40274 does not cover scaling issue on external monitor which is the most annoying part for me as it is my primary display.

[nyalldawson]

Should be all fixed in QGIS 3.32 -- please open new tickets for any specific individual hidpi issues remaining after this release