[Bug]: Moving the viewport using Ctrl+arrow keys is inconsistent

[kwvanderlinde]

Describe the Bug

When using the keybindings for moving the viewport by whole cells, the viewport is not only moved by a cell count but also snapped so that the top-left corner of the viewport lies exactly on a cell border. The direction of this snapping depends on the sign of the coordinates of the top-left corner, resulting in inconsistencies in situations that appear identical to the user.

To Reproduce

1. Create a blank map with a square grid. Bonus points for having a solid color background.
2. Drop a Medium sized token on cell (1, 1). This will serve as a visual reference.
3. Set zoom to 100% and drag the map so the top-left of the viewport is approximately at the center of cell (0, 0).
4. Press Ctrl+Left. Notice the viewport is moved up-left by about half a cell (alternatively, the map is moved down-right by that amount).
5. Now move the token to cell (-1, -1), and reposition the map so the center of cell (-2, -2) is about at the top-left corner. This should visually be similar to the setup in (3).
6. Press Ctrl+Left. Notice this time the viewport moves left by half a cell, but also moves down half a cell, unlike in (4).

Expected Behaviour

Snapping should be consistent regardless of where the top-left corner of the viewport is.

Screenshots

In steps (4) and (6), this is what the set up looks like:

MapTool Info

1.15.2

Desktop

Linux Mint 22

Additional Context

No response

[kwvanderlinde]

Going beyond the bug itself, my preference would be that we don't do any snapping for Ctrl+arrow, instead always moving by whole cells. In my mind, there's nothing particularly special about the top-left corner that we should aim to align it with a grid cell.

If that opinion isn't widely shared, then at least it would be nice if snapping only applied in the direction of movement. I.e., Ctrl+Left would not disturb the viewport vertically, only horizontally.