When running Wolfram Mathematica, I found newly opened windows would never be matched to their launcher on the icon-only task manager widget. By debugging a bit, I found out that X11 always reports NULL on the field res_class for Mathematica windows. I don't know how anomalous is this, but the very first check libtaskmanager does when attempting to find a matching launcher url is to check if classclass isn't null (line 543 of taskitem.cpp). By looking at the code, little can be done to find a suitable launcher if this test fails. I suppose this test could be changed to check if *both* classClass and className are NULL.

I haven't found any FOSS that creates windows with NULL res_class, so it may very well be Mathematica's fault.

Reproducible: Always

Steps to Reproduce:
1. Open Wolfram Mathematica (or any program that creates windows with NULL X11 res_class) 
2. Right-click on the window's icon on icon-only task manager plasmoid and select "Show A launcher when not running"
3. Close the window
4. Reopen the program
Actual Results:  
Newly opened window icon is placed on the "unknown launcher" place on icon-only task manager, after the known launchers.

Expected Results:  
Newly opened window icon should be placed over the previously created launcher.

Well, I have taken the liberty to create a tiny patch that attempts to solve the problem by relaxing the non-empty classClass condition on line 543 of taskitem.cpp. So far this seems to work just fine, and Mathematica windows are correctly matched to its launcher without any vissible side effects. I admit though, I have next-to-zero knowledge on the inner workings of libtaskmanager, so I suggest some care with this patch. 
---
From 364403a0b0032046da80e2aafb57e2cb8625de21 Mon Sep 17 00:00:00 2001
From: Thiago Martins <tcmartins@gmail.com>
Date: Wed, 21 Nov 2012 20:47:41 -0200
Subject: [PATCH] Try to use className for a window with null classClass
---
 libs/taskmanager/taskitem.cpp |   13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/libs/taskmanager/taskitem.cpp b/libs/taskmanager/taskitem.cpp
index e9933c8..5a7a79c 100644
--- a/libs/taskmanager/taskitem.cpp
+++ b/libs/taskmanager/taskitem.cpp
@@ -540,7 +540,7 @@ KUrl TaskItem::launcherUrl() const
     // we will keep on failing so the isEmpty()  check above is not enough.
     d->checkedForLauncher = true;
 
-    if (d->task && !d->task.data()->classClass().isEmpty()) {
+    if (d->task && !(d->task.data()->classClass().isEmpty() && d->task.data()->className().isEmpty())) {
 
         // For KCModules, if we matchd on window class, etc, we would end up matching to kcmshell4 - but we are more than likely
         // interested in the actual control module. Thereffore we obtain this via the commandline. This commandline may contain
@@ -558,8 +558,13 @@ KUrl TaskItem::launcherUrl() const
         KConfigGroup set(&cfg, "Settings");
 
         // Some apps have different laucnhers depending upon commandline...
-        QStringList matchCommandLineFirst = set.readEntry("MatchCommandLineFirst", QStringList());
-        if (matchCommandLineFirst.contains(d->task.data()->classClass())) {
+        QStringList matchCommandLineFirst = set.readEntry("MatchCommandLineFirst", QStringList()); 
+        if (!d->task.data()->classClass().isEmpty() && matchCommandLineFirst.contains(d->task.data()->classClass())) {
+            triedPid = true;
+            services = getServicesViaPid(d->task.data()->pid());
+        }
+        // Try to match using className also
+        if (!d->task.data()->className().isEmpty() && matchCommandLineFirst.contains("::"+d->task.data()->className())) {
             triedPid = true;
             services = getServicesViaPid(d->task.data()->pid());
         }
@@ -585,7 +590,7 @@ KUrl TaskItem::launcherUrl() const
         // So, Settings/ManualOnly lists window classes where the user will always ahve to manual set the launcher...
         QStringList manualOnly = set.readEntry("ManualOnly", QStringList());
 
-        if (manualOnly.contains(d->task.data()->classClass())) {
+        if (!d->task.data()->classClass().isEmpty() && manualOnly.contains(d->task.data()->classClass())) {
             return d->launcherUrl;
         }
 
-- 
1.7.10.4