Bug 486401

Summary:	Lagging/Slow scrolling
Product:	[Applications] kdiff3	Reporter:	jonaskarlsson
Component:	application	Assignee:	michael <reeves.87>
Status:	CONFIRMED ---
Severity:	normal	CC:	1i5t5.duncan, kde
Priority:	NOR
Version:	1.11.0
Target Milestone:	---
Platform:	Arch Linux
OS:	Linux
Latest Commit:		Version Fixed In:
Sentry Crash Report:
Attachments:	Video of bug

Description jonaskarlsson 2024-05-01 11:28:09 UTC

Created attachment 169072 [details]
Video of bug

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Lagging/Slow scrolling


STEPS TO REPRODUCE
1. Open KDiff3 and do any diff of two files
2. Scroll using the scroll wheel of your mouse
3. 

OBSERVED RESULT
Lagging/slow scrolling

EXPECTED RESULT
Smooth scrolling

SOFTWARE/OS VERSIONS
KDE Frameworks Version 6.1.0
Qt  Version 6.7.0 (built against 6.7.0)
The wayland windowing system

ADDITIONAL INFORMATION

Comment 1 Kai Uwe Broulik 2024-09-23 08:17:32 UTC

I can reproduce a rubberband effect under Wayland, when run as X application it instead just drops the frames. Wonder if that is an issue with kdiff’s painting.

Fwiw I made a patch for Qt Wayland to implement backingstore scrolling, so it only has to repaint a small region as you scroll, which should improve performance (when not using fractional scaling) albeit not entirely: https://codereview.qt-project.org/c/qt/qtwayland/+/528803

Comment 2 Duncan 2024-09-24 05:14:36 UTC

[CCing after having seen the bug mentioned on the kde-core-devel list, which I follow via gmane.io's list2news service.]

Confirming here too. 

gentoo
kdiff3 1.11.4
qt 6.7.2
frameworks live-git (using the gentoo/kde overlay ebuilds) updated Sep 18, reports 6.7.0
plasma live-git desktop, wayland by default, plasmashell reports 6.1.90

I ran kdiff3 with --platform xcb to run in xwayland for the X side test.

I'm on decade-old hardware, amdgpu graphics.  The scrolling lag was there on wayland but not /horrible/.  I ran kdiff3 wayland-native first (it was a fresh build/install so was testing that it'd even run, too)  and honestly didn't notice a lag -- I suppose I was subconsciously attributing it to a scrolling animation effect.

Only after running it under xwayland and seeing the faster scrolling there did I really notice the wayland-native scroll-lag.   Then I ran two windows, one wayland-native one xwayland, flipping between them and scrolling, and the wayland-native scrolling lag became /quite/ obvious.

So now CCing and we'll see where this goes.  I might try that qtwayland patch if I get time.