https://bugs.kde.org/show_bug.cgi?id=466373
Bug ID: 466373
Summary: Qt apps don't use wp-fractional-scale-v1 for hidpi
fractional scaling on Wayland
Classification: KDE Neon
Product: neon
Version: unspecified
Platform: Neon
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: Packages Testing Edition
Assignee: neon-b...@kde.org
Reporter: edoubray...@gmail.com
CC: j...@jriddell.org, neon-b...@kde.org, sit...@kde.org
Target Milestone: ---
SUMMARY
Qt client apps using system Qt 5.15.8 still use the old downscaling mechanism
to deal with fractional scaling, instead of wp-fractional-scale-v1, leading to
visual artefacts.
STEPS TO REPRODUCE
1. Set "Scale" to fractional eg 135% in Display Configuration
2. Open a Qt app
OBSERVED RESULT
App is upscaled to 2x then downscaled. This produces aliasing artifacts as
detailed previously here [1]
EXPECTED RESULT
Native scaling... like what is done by XWayland in "Apply scaling themselves"
mode.
See attached picture. On the left is XWayland (Platfom=xcb) and on the right
Wayland.
SOFTWARE/OS VERSIONS
KDE Neon based on ubuntu 22.04.2
(available in About System)
KDE Plasma Version: 5.27.1
KDE Frameworks Version: 5.104.0
Qt Version: 5.15.8
libqt5core5a Version: 5.15.8+p22.04+tunstable+git20230209.0323-0
python3-pyqt5 Version: 5.15.9+dfsg-1+22.04+jammy+unstable+build21
ADDITIONAL INFORMATION
I believe the previous scaling is offered by Qt since 5.6 and is described here
[2] and [3].
And the new one was announced by Nate here [4].
I think the KWin part landed:
```
> wayland-info | rg frac
interface: 'wp_fractional_scale_manager_v1', version: 1, name: 10
```
but maybe the Qt patches were not added to Neon's patch collection? Is this [5]
related?
how can I verify if the Qt5 client libs have support?
I attach the small PyQt sample [6] used to show that the scale given to the app
is 2x and the screen geometry and physicalDotsPerInch is fake (dpi 115.834
instead of 163), with system Qt 5.15.8.
[1]:
https://www.reddit.com/r/kde/comments/lficfe/wayland_fractional_scaling_may_be_sort_of_a/
[2]: https://www.qt.io/blog/2015/01/27/qt-weekly-24-high-dpi-qtwayland
[3]: https://www.qt.io/blog/2016/01/26/high-dpi-support-in-qt-5-6
[4]:
https://pointieststick.com/2022/12/16/this-week-in-kde-wayland-fractional-scaling-oh-and-we-also-fixed-multi-screen/
[5]: https://invent.kde.org/qt/qt/qtwayland/-/commit/23d3fc7
[6]: https://gist.github.com/eddy-geek/2a682a415abb5d351537ae3fc865dffd
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