Created attachment 202707 [details]
A file which can be used to reproduce the bug

If one manages to create a selection list via the Validity dialog (which is a bit broken at the moment in itself (see bug 156263), the selection list works fine.

However, sometimes after selecting any entry on the selection list, movement with the cursors will be broken. Either one of these symptoms will happen:
- Cursors will leave a trail of violet borders in only one direction, i.e. one can only move into one direction leaving this "trail" or back, deleting the "trail" of outlined cells
- There will be a green outline on the cell with the selection list, and with cursor keys one can move either one cell to the left, right, up or down from this cells.

The bug can be reproduced but will require some effort. I'm not sure what the actual trigger is but it's triggering often enough here by accident to become a nuisance.

(Saving and) reloading the document will "reset" the cursor key navigation back to normal.

I will try to capture a video recording of the bug later as this is indeed a bit difficult to describe, and possibly reproduction may require some usage pattern / mannerism not all users might share. It may require precise mouse movement or something, but as I said, I tried various ways but can not reproduce at will.

Steps to reproduce (Open LO Calc):
- Create some cells you wish to use for a source of list of selections
- Click on a cell you want to be able to choose from this list of selections
- Open Data -> Validity... -dialog box
- Choose Allow: Cell Range
- Add something into "Source" -field. (One should be able to select a range by mouse, but this functionality is broken here as per bug 156263). Type a range manually referring to the cells you created in 1st step. Note if bug 156263 triggers, your LO instance UI may be in a half-broken state from hereon. I will send a test case later up to until this step. If you've created a test case as per these instructions, you could save the file, kill LO process (as it can not be closed normally) and reload the file. 
- Try to select any choice from the drop-down menu you've just created
- Move around with cursor keys
- If you can move properly, retry two previous steps.

Observed results:
- Weird purple and green outlines around the cell with the selection list, and cursor key navigation utterly broken around these outlines.

Expected results:
- Selection should happen as expected and cursor keys should work as expected after making a selection

Version: 25.8.1.1 (X86_64) / LibreOffice Community
Build ID: 580(Build:1)
CPU threads: 32; OS: Linux 6.16; UI render: default; VCL: kf6 (cairo+xcb)
Locale: fi-FI (fi_FI.UTF-8); UI: en-US
25.8.1-1
Calc: threaded