As the title suggests, my Alt+Tab Application Switcher only shows a generic icon for some applications, mostly Mate programs.

My main question is, What directories does compiz search for the icons that are displayed in the Application Switcher?

Atril has its icon displayed properly, but mate-terminal, mate-calc, pluma, and others do not. Looking at the .desktop files, if that's what is used, the icon requested for atril resides in the 'hicolor' directory but the others seem to be in 'mate'. Changing the icon theme in the Mate Appearance dialog between themes does not help either.

FWIW this problem does not happen in compiz 0.9.x.

This might be a bug, but I don't know how to further diagnose the problem. Any suggestions?

Just to follow up, I see there is a pretty long-standing bug on launchpad for this (#1088178). To investigate further, I compiled both versions of compiz (0.8 and 0.9) to separate prefixes and ran them in different user accounts, and both nowadays exhibit this behavior. I in fact compiled the exact same 0.9 tarball that I used on BLFS-7.9, where I did not have this problem.

There is not much documentation on this, but after poking around in the source, it looks like compiz gets its icons from X11 and not through the *.desktop files directly. In short there is miscommunication between compiz and X11 nowadays that did not happen before, so this is unlikely a compiz problem.

To investigate, you can launch an offending program for which compiz gives you a generic icon by using strace and directing the output to a text file, and then closing the program. In the log, look for icons, specifically 'png' files, and note which icon and size is first listed.

Now run xprop from a terminal (this should be included in a standard X11 installation). The cursor should turn into a + symbol. Launch the offending program, and then click with the + pointer on the program window (the actual window, not the title bar). Scroll up through the output and verify that icons are displayed.

In my case, the first icon found by strace was of size 256x256, and xprop showed (not displayed) next to this size.

I further found that I got compiz to show all icons properly when I used an icon theme called ContrastHigh-SVG that is apparently included with Mate.

Then, I compared the index.theme files for the 'Mate' and 'ContrastHigh-SVG' themes. Viola. ContrastHigh-SVG does not contain any references to 256x256 icons, and the largest is 128x128.

So, I backed up the index.theme and icon-theme.cache files for the Mate icon theme, opened index.theme and removed all references to 256x256 in the "Directories" line as well as the 256x256 entries farther below. Then I ran gtk-update-icon-cache to regenerate the cache. In the Mate Appearances I switched the icon theme to something else, and then back to my usual theme. Press ALT+TAB and all icons show up properly.

I'm not marking this as solved as this is only a workaround.