[okular] [Bug 518664] New: Laggy smooth scrolling animation under GNOME wayland session

[Tianyou Zeng]

https://bugs.kde.org/show_bug.cgi?id=518664

            Bug ID: 518664
           Summary: Laggy smooth scrolling animation under GNOME wayland
                    session
    Classification: Applications
           Product: okular
      Version First 25.12.3
       Reported In:
          Platform: Arch Linux
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: okular-devel at kde.org
          Reporter: timtyzeng at gmail.com
  Target Milestone: ---

SUMMARY
When smooth scrolling is enabled in Okular, scrolling via the keyboard (Up/Down
arrows, J/K keys) or the mouse scroll wheel randomly results in a laggy,
stuttering, or sometimes completely skipped animation.

This issue appears to be strictly isolated to the GNOME Wayland session. The
scrolling animation is perfectly smooth in GNOME X11, KDE Plasma Wayland, KDE
Plasma X11, and even Windows 11 (using the binary built by Craft).

STEPS TO REPRODUCE
1. Log into a GNOME Wayland session and open a PDF document in Okular.
2. Ensure that the "Smooth scrolling" option is enabled in Okular's settings.
3. Scroll through the document by pressing/holding the Up/Down arrows or J/K
keys, or by using the mouse scroll wheel.

OBSERVED RESULT
The brief scrolling animation drops frames, appearing laggy, discontinuous, or
sometimes skips the animation entirely (jumping instantly to the next
position).

EXPECTED RESULT
A consistently smooth scrolling animation that matches the monitor's refresh
rate (e.g., 60Hz).

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
(available in the Info Center app, or by running `kinfo` in a terminal window)
Linux/KDE Plasma: Arch Linux / GNOME Wayland
KDE Plasma Version: N/A
KDE Frameworks Version: 6.6
Qt Version: 6.11

ADDITIONAL INFORMATION
The scrolling animation in Okular is invoked via the QScroller::scrollTo()
method. To isolate the problem, I wrote a minimal Qt application testing the
QScroller class under GNOME Wayland, and it worked perfectly smoothly. This
suggests the issue is not a bug within QScroller itself.

However, I suspect the root cause lies in an edge-case incompatibility between
how Qt APIs and the GNOME Wayland compositor. Before the issue is identified
and resolved upstream by Qt or GNOME, it would be ideal to implement a
temporary, low-impact workaround in Okular's codebase to resolve the stuttering
without affecting general functionality or performance. This patch could then
be cleanly reverted once the underlying upstream issue is fixed.

I will continue investigating this and may try to implement the fix myself and
submit a pull request later.

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