XnView Software

Okay, but this still looks like an XnView issue, because somehow it's managed to set itself up to open these files, but not to do so properly.

I don't have any other applications that can open them, and I haven't edited the registry directly.

Dragging an image onto the icon works. It's just the double-clicking to launch the Open action that doesn't.

Yes, or launching from another shell-like application (e.g. an Everything search).

No, I didn't add, remove, or change any plugins at all.

I have set XnView MP to open a window (not the browser and not fullscreen) when I open a file. However, when I double-click an HEIC file in Windows 10 to view it in XnView MP, it opens XnView MP as expected, but the window is blank with "< Empty >" in the title bar.

If I then drag the HEIC file ...