I have another backtrace, the only difference is in the first lines:

(gdb) #0  0x0018847b in KWinInternal::Client::windowType ()
   from /usr/lib/libkdeinit_kwin.so
#1  0x001885db in KWinInternal::Client::isDialog ()
   from /usr/lib/libkdeinit_kwin.so
#2  0x00188750 in KWinInternal::Client::wantsTabFocus ()
   from /usr/lib/libkdeinit_kwin.so
#3  0x00198ed2 in KWinInternal::Workspace::CDEWalkThroughWindows ()
   from /usr/lib/libkdeinit_kwin.so
#4  0x0019a062 in KWinInternal::Workspace::slotWalkThroughWindows ()
   from /usr/lib/libkdeinit_kwin.so
#5  0x0018541b in KWinInternal::Workspace::qt_invoke ()

...

Both the backtraces clearly show KWin is in code for the Alt+Tab dialog, so they disagree with your description of the problem.

Your observation shed more light on the problem. By trying with my alt+tab combination, which is win+tab, I finally discovered a way to reproduce the bug on my system. I often use a single full-screen window per workspace, and I have a combination to close the window, and the bug often manifested after having closed a window, but now I have only been able to reproduce the bug with the alt+tab combination so may be I incidentally press alt+tab (maybe to check if there are other windows below the full-screen one) and do not notice that.

However, to reproduce the bug, I press alt+tab, or win+tab, or whatever key combination I assign to it, and then, even after some time (say 5-6 seconds) I click on the desktop. This triggers the bug. I already tried deactivating khotkeys and also the K menu.

WORKAROUND:

The bug does not show if I choose "show window list while switching windows". Also, I found that the bug shows up even in Xnest and that the bug sometimes (but not always) does not show up if I manually start kwin, kicker, kdestkop, korgac, klipper from the command line instead of using startkde.

This is a grave bug, leading to the entire computer being unusable in such a way that a normal user can difficultly understand what happened, and I think that this bug should at least be _confirmed_. It's still there in kde 3.5.

It's easy: just go to kcontrol, then

1) Desktop->Window Behaviour->Deselect "Show Window list while switching windows", ok

2) Close all your windows

3) Press ALT+TAB, release alt+tab

4) Click on the empty desktop

In kde 3.5 you can still ctrl+alt+backspace, luckily.

Is it just my problem on three different machines, or is there anyone else who can confirm?

SVN commit 488474 by lunakl:

Fix a nasty infinite loop with CDE-style Alt+Tab.
BUG: 112737



 M  +18 -17    tabbox.cpp  


--- branches/KDE/3.5/kdebase/kwin/tabbox.cpp #488473:488474
@@ -932,28 +932,29 @@
         options_traverse_all = KGlobal::config()->readNumEntry("TraverseAll", false );
         }
 
-    if ( !forward )
+    Client* firstClient = 0;
+    do
         {
-        do
+        nc = forward ? nextStaticClient(nc) : previousStaticClient(nc);
+        if (!firstClient)
             {
-            nc = previousStaticClient(nc);
-            } while (nc && nc != c &&
-                (( !options_traverse_all && !nc->isOnDesktop(currentDesktop())) ||
-                 nc->isMinimized() || !nc->wantsTabFocus() ) );
-        }
-    else
-        {
-            do
+            // When we see our first client for the second time,
+            // it's time to stop.
+            firstClient = nc;
+            }
+        else if (nc == firstClient)
             {
-            nc = nextStaticClient(nc);
-            } while (nc && nc != c &&
-                    (( !options_traverse_all && !nc->isOnDesktop(currentDesktop())) ||
-                     nc->isMinimized() || !nc->wantsTabFocus() ) );
-        }
-    if (c && c != nc)
-        lowerClient( c );
+            // No candidates found.
+            nc = 0;
+            break;
+            }
+        } while (nc && nc != c &&
+            (( !options_traverse_all && !nc->isOnDesktop(currentDesktop())) ||
+             nc->isMinimized() || !nc->wantsTabFocus() ) );
     if (nc)
         {
+        if (c && c != nc)
+            lowerClient( c );
         if ( options->focusPolicyIsReasonable() )
             {
             activateClient( nc );

*** Bug 118758 has been marked as a duplicate of this bug. ***