Some textures (milky way) moving at different rates than stars with fractional scaling

[GuLinux]

Screencast_20250118_195227.webm

Expected Behaviour

The milky way/deep sky texture should be fixed with the stars, and move at the same rate when panning, regardless of desktop scaling settings.

Actual Behaviour

When desktop scaling is set to a fractional scale value, stars and milky way move in different speed/directions when panning (see attached video).

Steps to reproduce

Issue encountered on Arch Linux with KDE, Wayland and fractional scaling enabled (1.5), with both intel and nvidia drivers (latest versions).
With scaling set to either 1 or 2 the effect disappear.
Note that if the desktop is set to 1.5 scaling, even starting the app with QT_SCALE_FACTOR=1 or QT_SCALE_FACTOR=2 does not fix the issue.

System

• Stellarium version: 24.4
• Operating system: Arch linux
• Graphics Card: several tried (intel integrated GPU, NVIDIA GeForce GTX 1650, NVIDIA GeForce RTX 4070)
• Screen type: 2560x1600,, HighDPI, scaling 1.5

[10110111]

Note that if the desktop is set to 1.5 scaling, even starting the app with QT_SCALE_FACTOR=1 or QT_SCALE_FACTOR=2 does not fix the issue.

What if you use QT_SCREEN_SCALE_FACTORS instead of (or in addition to) QT_SCALE_FACTOR? And also maybe set QT_FONT_DPI=96, this variable also affects the scaling, with 96 being the default value for low-res screens.

[GuLinux]

Hi, thanks for your reply. No effect for any of them, I've tried various combinations 🤷

[10110111]

Completely no effect, even no changes to font size? I wonder how we can force a Qt scaling factor on Wayland then. Does this happen on X11?

[GuLinux]

If I set everything to scale 2x then I do see the fonts change, yes. But the bug is still there.

If I set it to 1 instead, it looks like there's no change at all, not even fonts size.
I can't try on X11 as I don't have an X server installed at the moment (though not sure how relevant it is, as it doesn't support fractional scaling anyway)

[gzotti]

There seem to be numerous other projects suffering from the same problems around "Qt/Wayland". A few random picks:

https://forum.garudalinux.org/t/solved-qt5-qt6-wayland-scaling-dpi/18041
https://forums.ankiweb.net/t/blury-interface-with-gnome-wayland-and-fractional-scaling/19196/6

Is this with Qt5 or Qt6? Seems to make a difference.

[GuLinux]

Qt6:

ldd /usr/bin/stellarium 
        linux-vdso.so.1 (0x000076a6c4760000)
        libQt6WebEngineWidgets.so.6 => /usr/lib/libQt6WebEngineWidgets.so.6 (0x000076a6c46f2000)

Exporting QT_QPA_PLATFORM=xcb as suggested in the first link manages to suppress the issue, thanks