I posted this topic on Arch Linux, and got a reply related to Fvwm.
When Picom compositor gets loaded, the tiling is removed. The shadow and other effects still work fine.
The compositors pick the wallpaper pixmap from the ESETROOT_PMAP_ID and/or _XROOTPMAP_ID properties on the root window.
Fvwm probably doesn’t set a full-size, pre-tiled pixmap there, but only the tile.
For Picom, the solution is to replace
fvwm-root -r with
feh --bg-tile, and it works well. Requires to install feh package. Any better solution??
I would just use
feh, it is a far better tool anyways, it not only supports more image formats, it can stretch/scale images to fit the background too.
I really ought to remove fvwm-root, it’s not something I want to maintain anyway,
It works on Debian-based distros but on others it’s conflicting. I had a look at RPM Fvwm3 package on different distros, and feh is added as a dependency. Fvwm-root, fvwm3-root and other limitations make it obsolete. Feh does the job and has more options.
That’s completely irrelevant. The point here is that external tools such as feh(1) or nitrogen(1) do a far better job – and all the while that fvwm-root exists, it’s making an assumption that it works and is maintained. In reality, that’s just not true.
As for the differences between distros, that’s out of scope here.