Well, after going through and comparing files in my ~/.config/pcmanfm/ folders with those in /home/test/.config/pcmanfm/, the only differences I found were:
1.) How the wallpaper was handled
2.) how window manager menus were displayed.
(The first of these may take us back to the suggestion made by Ferdes Fides much earlier in the thread.
I still believe the eventual solution in my case will be slightly different from his, but they might share a "common ancestor".)After backing up my original
~/.config/ folder (as the affected user), I logged in as root and deleted the affected user's folder ~/.config/pcmanfm/
. (NOTE: don't delete ~/.config/, as there is a lot of other stuff besides pcmanfm there - this will have wide-ranging impact on your system.)
I logged out as root (didn't want to do the deletions with the affected account still open), logged back in as the affected user, and noticed, of course, that my wallpaper had reverted to the single default wallpaper - the contents of ~/.config/pcmanfm were re-created (from /etc/skel/ if I'm right) and all the default desktop shortcuts were added to those I had created locally (I had previously gotten rid of some of these).
The annoying business where I had to open the configuration menu and double-click the Set Wallpaper icon to display my desktop (instead of the "plain brushed grey background" I'd mentioned earlier) has been corrected!
PCManFM starts as it should from the menu, and from a .desktop item I created by sending that menu item to the desktop!
This leads me to believe that something in the old ~/.config/pcmanfm/ folder was, indeed the culprit here. (I wonder if it could be a stray space or other character causing a "parse error" when the file is read on log-in?) It appears from the above that deletion of the affected user's ~/.config/pcmanfm/ folder was the "quick fix" here, but I'm a long way from done with this yet.
(I still want to identify the root cause, if possible.)
I noticed something odd, and feel it's worth reporting. On log in and determining that PCManFM worked, I tried to launch Dolphin, KTorrent, and K3B, but none of them would launch. (All had been working previously.)
"Hmm...," says I, then, I wonder for a moment and decide to try a restart of the computer. Now, after logging back in, everything works. (PCManFM, Dolphin, K3B, KTorrent all launch and run properly.)
Now there are a few more items on my "to do" list:
1.) Attempt to set up individual wallpapers again and see if my problem recurs with this new configuration. (I'll test after each change
2.) If this new configuration works properly after I have got my desktop configured as I want it, back up the ~/.config/ folder again.
3.) Restore the ~/.config/pcmanfm/ folder from my PREVIOUS backup and see if I can make the problem recur. (I expect it will...)
4.) Compare the restored original folder's contents with those of the backup taken in step (2.) above to attempt to identify the bad actor(s).
Well, I'd better get crackin' if I want to get all this done tonight. I'll jot back with what I find out.
DP.S. I don't even have to setup a second wallpaper, just enable different wallpapers (uncheck the box in Set Wallpaper for "use the same wallpaper for all desktops") and it fails. Now for a post-mortem on the .config/pcmanfm/ folder to see what changed.
Odd things: in the file ~/.config/pcmanfm/LXDE/pcmanfm.conf, I had set desktops 1 and 2 to have different wallpapers. When I logged back in, it appears that the system "fell back" to ~/.config/pcmanfm/default/pcmanfm.conf and loaded the single default wallpaper lxde-blue.
Meanwhile, the ~/.config/pcmanfm/LXDE/pcmanfm.conf file had wallpaper entries for wallpaper1 and wallpaper7 (no entry for wallpaper0). I corrected this manually, making the wallpaper7 entry as the wallpaper0, and tried again (logged in and out). The desktop still did not appear, presenting the plain grey brushed background, until I opened the configuration menu and double clicked on Set Wallpaper. PCManFM still fails to launch as in my initial symptoms.
I copied the edited ~/.config/pcmanfm/LXDE/pcmanfm.conf file over the ~/.config/pcmanfm/default/pcmanfm.conf file, with the same result, except now the wallpapers are as I set them.
I'm beginning to believe that the failure of PCManFM to start is just a side effect, and this failure of the desktop to display on log in may be the real problem, but I ain't no-how sure any more. I'm going to let this stew for a while before proceeding further in the hope that one of you may have some insight, or that some time away from it might help me break something loose.