Jagged grid lines artifacts with fractional scaling

[codeandfire]

My laptop’s screen is FHD (1920x1080) and 15.6" diagonally. This is not "very HiDPI" (as in, requiring scaling of 200% or more) but still requires some scaling of around 140%.

KDE Plasma has support for fractional desktop scaling. To summarize what it does:

• You can use a scaling factor that is not a whole integer such as 1 or 2, but a fractional number between 1 and 2 (e.g. 1.4).

• The entire desktop gets universally scaled.

• It works on both X11 and Wayland.

Desktop scaling with KDE Plasma has worked smoothly for me for all my apps so far. Only in Ipe there is this glitch — the grid lines develop jagged artifacts as my mouse cursor hovers over them.

Notably the web version of Ipe shows no such artifacts, even though the same desktop scaling factor is still there.

I’ve taken two screen recordings — I’ve used the http://0x0.st pastebin, and both files are < 1 MiB in size (so they should not be a problem to download and play).

• Jagged artifacts in desktop app

• No artifacts in web version

The artifacts show up in the desktop app on both X11 and Wayland (no distinction there).

• Operating system: Debian 13 (trixie)

• KDE Plasma version: 6.3.6

• Qt version: 6.8.2

• Ipe version: 7.2.30

• GPU: Hybrid (integrated Intel GPU + dedicated NVIDIA GPU)

I think the issue is related to Qt and not KDE Plasma, because if I turn off the desktop scaling (make it 1 instead of 1.4) and use QT_SCALE_FACTOR instead, I still get the same artifacts:

$ QT_SCALE_FACTOR=1.4 ipe

If I use an integer value of QT_SCALE_FACTOR such as 2, the artifacts don’t appear, which suggests that the issue is with fractional scaling and not integer scaling.

I also don’t think the issue is related to the hybrid GPU, because I tried booting a Debian 13 / KDE Plasma live environment on another machine with only an integrated Intel GPU, and I can again see the same artifacts on both X11 and Wayland in Ipe.

Lastly, a few months back, I had tried Ipe on a Linux Mint 22.1 (Xia) system with Cinnamon as the desktop environment. Cinnamon has no support for fractional desktop scaling (it’s labelled "experimental" and doesn’t work well in practice). You can only set a text scaling factor to scale only font (not other elements such as icons). With the text scaling factor set to 1.5, I recall that Ipe showed the same jagged artifacts. So, that may be another point worth noting.

I've read this FAQ entry and this issue but what I see in my case is that Ipe has responded perfectly in terms of the UI (menus, toolbars etc.) to the scaling factor - it is only the grid lines that are showing such artifacts.

Ipe is quite unusable with these odd-looking grid lines. I can use the web version, but I would really like the desktop app to work! Thank you very much for Ipe!