[okular] [Bug 418086] New: Okular starts over rendering preview of page when scrolling

[Postix]

https://bugs.kde.org/show_bug.cgi?id=418086

            Bug ID: 418086
           Summary: Okular starts over rendering preview of page when
                    scrolling
           Product: okular
           Version: 1.9.2
          Platform: Other
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: okular-devel at kde.org
          Reporter: postix at posteo.eu
  Target Milestone: ---

Created attachment 126334
  --> https://bugs.kde.org/attachment.cgi?id=126334&action=edit
Screen recording

SUMMARY

When I opened a PDF (21 MB) with huge images, which took some time to render, I
noticed that the preview first starts to be generated after the actual page has
been rendered completely. 

This could probably be done in a second thread simultaneously or the preview
could make use of the already rendered page.

However, it seems that, as long as the preview rendering did not finish, the
preview is rendered from the very beginning whenever the user scrolls a little
bit in the PDF -- a few pixel or a scroll event seem to be enough.

Please take a look at the screen recording. The preview starts rendering at T =
30 s.

SOFTWARE/OS VERSIONS
Operating System: Manjaro Linux 
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.67.0
Qt Version: 5.14.1
Kernel Version: 5.5.2-1-MANJARO
OS Type: 64-bit
Processors: 4 × Intel® Core™ i7-7500U CPU @ 2.70GHz
Memory: 15,4 GiB

Additional Information:
RAM usage: Greedy
Rendering: Anti-Aliasing, Hinting, Transparency Effects

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