XnView Software

XnView: MP 1.3.1-1.4.0b1 64bits
OS: Windows 64bit (22H2), display resolution 3840x2160, win scaling 150%

Filter
pressing Undo applies current filter

To reproduce:
1. run clean portable XnViewMP
2. open any image
3. open Filter > Filter.. dialog
4. select any filter, e.g. Average, (optional) maximize its settings for better visibility
5. press Undo button

Issue A
filter applies to image
6. press Cancel to close Filter dialog
filter still applied to image

Issue B
6. press Cancel to close Filter dialog
7. press left/right to scroll to previous/next file in folder
8. scroll back to initial file
filter still applied to image
issue caused by Viewer cache and disappears when 'Keep current image' checkbox is unticked (View > Misc > Cache)

Thanks to your detailed description I can reproduce

See issue for current status and some details.

This problem is supposed to be fixed in XnView MP 1.4.3. Please check and confirm the bug fix here.

1.4.3
Apply and Undo buttons are always disabled now,
guess it can be counted as fixed