[okular] [Bug 407549] New: On continuous view, if >2 pages fit on-screen, left/right arrows inconsistent (next/previous page)

[jimbo1qaz]

https://bugs.kde.org/show_bug.cgi?id=407549

            Bug ID: 407549
           Summary: On continuous view, if >2 pages fit on-screen,
                    left/right arrows inconsistent (next/previous page)
           Product: okular
           Version: unspecified
          Platform: Other
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: okular-devel at kde.org
          Reporter: jimbo1qaz at protonmail.com
  Target Milestone: ---

SUMMARY
If I enable continuous mode and zoom out until over 2 pages fit on screen at
once (eg. widescreen PDF slides), then scrolling computes page number using
center of screen, but left/right arrows aligns selected page to top of screen.

STEPS TO REPRODUCE
1. Open a PDF document and zoom out until over 2 pages fit on screen.
    - The bottom of the screen says page 1 (top of screen).
2. Press right arrow to enter page 2.
    - The bottom of the screen says page 2 (top of screen).
3. Scroll down and up.
    - The position of the document is identical to "after step 2", however...
    - The bottom of the screen says page 3 (center of screen).

If you press left/right arrow after step 2, you will end up on page 1/3.
If you press left/right arrow after step 2, you will end up on page 2/4.

EXPECTED RESULT
I'm not sure how to fix this behavioral inconsistency. I personally want a
choice in the Settings menu between multiple different behaviors, but you can
hard-code one of the behaviors, or find another internally-consistent and
user-friendly solution.

## Top of screen

`top_float` = "page number at top of screen" (similar to left/right arrow
today). Defaults to 1.0 when opening a document. Scrolling half of page 1
off-screen = 1.5.

I want pressing (left, right) for the following `top_float` to set `top_float`
to:

2 -> (1, 3)
2.01 to 2.99 -> (2, 3)

- Left arrow moves to page `ceil(top_float) - 1`
- Right arrow moves to page `floor(top_float) + 1`
- The page number is displayed as `floor(top_float)`

## "Centered Page" in scare-quotes

`top_float` = "page number at top of screen" (as above).
`center_float` = "page number at center of screen" (similar to Okular scrolling
today).

I want pressing (left, right) for the following `top_float` to set `top_float`
to:

1.5 to 2.49... -> (1, 3)
2.5 to 3.49... -> (2, 4)

- Left arrow moves to page `round(top_float) - 1`???
- Right arrow moves to page `round(top_float) + 1`???
- The page number is displayed as `round(top_float)` (matches left/right
arrows).

Currently, Okular uses this approach, but with `floor(center_float)` instead,
which suffers from the above-mentioned consistency issues.

------

A previous version of Windows Foxit used "top of screen" behavior for
left/right arrows (which I disliked at the time), but an update changed it to
"center of screen" behavior (and now I want the older behavior back XD).

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 18.04
(available in About System)
KDE Plasma Version: 5.12.7
KDE Frameworks Version: 5.44.0
Qt Version: 5.9.5

ADDITIONAL INFORMATION
Okular versions with bug:
Ubuntu: 1.5.70
Flatpak flathub: 1.7.0
Flatpak kdeapps: 1.7.70 master, commit f741df1186ad

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