[Libreoffice-bugs] [Bug 81829] Viewport calculations for .uno:PageUp .uno:PageDown scroll movements are not accurate (comment 28)

[bugzilla-daemon at bugs.documentfoundation.org]

https://bugs.documentfoundation.org/show_bug.cgi?id=81829

--- Comment #37 from V Stuart Foote <vstuart.foote at utsa.edu> ---
No the reason for reopening this is only to review the calculation of the
viewport(s) and why repositioning of the view port is not consistent--either
when positioned at the top of the document canvas and first use of
.uno:pageDown, or with subsequent scrolling movements up or down with the UNO
controls.

The viewport height and width is static as set by the current zoom scale of the
VCL canvas. And movements of the viewport with UNO controls linked to pgUp or
pgDn buttons should be in consistent amounts relative to the zoom scale.

Ideally the movement range would consistently reposition the viewport some
fixed percentage of the VCL canvas and the zoom scale--and I thought that was
being calculated at 9/10's in the edit shell at:

https://opengrok.libreoffice.org/xref/core/editeng/source/editeng/impedit2.cxx?r=dc37866f&mo=53745&fi=1411#1398

and that the viewport not moving in that increment needs attention.

The new (for bug 105776) <Alt>+<pgDn>/<pgUp> UNO controls (
.uno:GoToStartOfNextPage / .uno:GoToStartOfPrevPage ) do not work against the
viewport, they are movements against the document structure.  They inherently
work in full page increments at any zoom scale. And they were long overdue.

But here we need to figure out why the viewport scrolling  movements are not
well constrained--there could be differences by os/DE, but seems like it should
be consistent.  Viewport scrolling should depend just on the VCL zoom scale,
the current view port Y-height and X-width, and what ever advance percentage is
used to calculate the top left and bottom right of the current viewport--and
then the values for the following. Consistent either scrolling up or down.

IIUC the Smooth scroll function in Writer comes into play about here as well.

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