XnView Software

XnView: MP 1.9.5 64-bit
OS: Windows 11 64-bit

My settings (important for reproducing the bug):
General > ENABLED Allow only one application instance
Interface > ENABLED Allow multiple Viewer tabs for same file
XnView MP as associated program

To reproduce:
1. Open photo in full screen
2. Context menu > Open with > Associated program OR press F3

Actual behaviour (bug):
1. Appear menu bar, toolbar and status bar, but program is still in full screen.
2. Press F11 to exit full screen - opened tab is empty (but thumb and filename on tab is correct).
3. Click on first tab - only title bar and tab bar is showing.
4. Close XnView and open it again - title bar is missing and it stay like this.
5. Entering and exiting full screen restores title bar.

Expected behaviour: Exit full screen and normally open file again in new tab (like XnView Classic do). Alternatively open file again in new tab in background (without full screen leaving).

Thanks for reporting this detailed bug!
I was able to reproduce the same issue on XnView MP 1.9.5 (Windows 11, 64-bit). The title bar and toolbar behave exactly as you described — disappearing after reopening the app following full screen mode.

It seems related to how XnView MP handles the “Open with associated program” command when multiple viewer tabs are enabled. Hopefully, the devs can fix this in the next update.