[okular] [Bug 480036] New: Zooming with CTRL + Mouse Wheel scroll results in wrong zoom levels with high performance hit

[Stiv]

https://bugs.kde.org/show_bug.cgi?id=480036

            Bug ID: 480036
           Summary: Zooming with CTRL + Mouse Wheel scroll results in
                    wrong zoom levels with high performance hit
    Classification: Applications
           Product: okular
           Version: 23.03.80
          Platform: Arch Linux
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: okular-devel at kde.org
          Reporter: stiv02570 at hotmail.com
  Target Milestone: ---

SUMMARY
Moving from Okular 22.12.3 to 23.03.8 using CTRL + Mouse  wheel scroll to zoom
in or out no longer follows the preset zoom percentages (100; 150; 200; 400;
...; 10,000), but zoom in is always +24 % and zoom out is always -24 % of the
*current* value.


STEPS TO REPRODUCE
1. Use any okular version after 22.12.3
2. Hold CTRL + Scroll to zoom
3. Get very uneven zoom percentages after multiple zoom ins and outs

OBSERVED RESULT
Having the uneven zoom percentages (triple zoom in from 100 % zooms in
consecutive +24 % of the current value, resulting in 190,7 % zoom) causes
severe performance drops at high zoom levels. Rendering at high zoom levels
takes many seconds (tested on multiple systems, high-end Hardware suffers just
the same), but using the CTRL + "+ key" results in the usual steps (...2,500 %;
5,000 %; 10,000 %) renders the content nearly instantly. The relative to
current value zoom levels also skip the [Fit Page] and [Fit Width] levels of
zoom.

Since it is always + or - 24% of the current value, it is impossible to return
to the "original" zoom level when alternating zooming in and out, ultimately
one gets a reduced zoom level. For example, starting at 100 % and zooming in
and out 30 times in a row lands you at 41.1 % zoom level.

EXPECTED RESULT
Using CTRL + Mouse Wheel to zoom should jump between the preset zoom levels
(100, 125, 150, 200, 400,...) the same way when using CTRL + "+ key", as it was
behaving up until Okular version 22.12.3.

SOFTWARE/OS VERSIONS
Arch Linux, Kernel-Version: 6.7.0-arch3-1
KDE Frameworks Version: 5.114.0
Qt Version:  5.15.12 (built against 5.15.12)

ADDITIONAL INFORMATION
Checking the dates of the releases between the Okular versions 22.12.3 and
23.03.8 the only two commits mentioning any changes to zoom or scroll in this
timeframe are 30969b49 (Feb. 20th, 2023) and 0b25a83c (Feb 20th, 2023).

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