[okular] [Bug 504458] New: On a scaled output, lines of pixels are sometimes missing or duplicated

[Éric Brunet]

https://bugs.kde.org/show_bug.cgi?id=504458

            Bug ID: 504458
           Summary: On a scaled output, lines of pixels are sometimes
                    missing or duplicated
    Classification: Applications
           Product: okular
      Version First 25.04.0
       Reported In:
          Platform: Other
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: PDF backend
          Assignee: okular-devel at kde.org
          Reporter: eric.brunet at lps.ens.fr
  Target Milestone: ---

Created attachment 181452
  --> https://bugs.kde.org/attachment.cgi?id=181452&action=edit
photos of the screen

When rendering a pdf, there are sometimes a complete line of pixels missing or
being duplicated. This makes text blurry, and sometimes changes its meaning (a
missing line of pixel might change a = into a - in a math document...).

I am attaching two photos of the screen in the file bug.jpg. On the left, the
bug is visible four times: there are two duplicated pixel lines in the third
line of text (check the word "consectetuer"), and two missing pixel lines in
the fourth line of text (check the word "pretium": the top is shaved, and the
horizontal line in the e is too small). The right part of bug.jpg shows the
same text correctly rendered.

I am using kde on an uptodate fedora 41 distribution, with screen scaling
enabled at 125%. If I set screen scaling at 100%, I don't see the bug anymore.
I see the bug quite easily with xorg; it happens also with wayland, but much
less frequently. The bug happens when scrolling the text within a fixed window,
on the laptop screen (there are no external monitor plugged in). One can
usually make the bug disappear by scrolling around or playing with zoom levels.
When I took the attached photos, it was enough to give the focus to another
application to make the bug disappear: clicking on another window seemed to
force a redraw of okular (why?) with no extra or missing lines.

This is mostly horizontal lines, but I have also seen, much less frequently,
missing vertical lines of pixel.

fedora 41
plasma and kwin 6.3.5
wayland 1.23
qt 6.8.2
xorg 21.1.16
okular 25.04
poppler 24.08

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