Created attachment 50625 [details]
Screenshot

This shows how the panel become after trying to resize it. It is in its maximum allowed width.

Created attachment 50627 [details]
plasma-desktop-appletsrc

This is a config with the problem.

I removed the panel and added it again. The new panel does not have the same problem. However, I think that this problem can affect other users that migrate from 4.4 to 4.5.

I deleted all ~/.kde/share/config/plasma* files and restarted KDE. The panel got the correct width; however, as soon as I tried resizing it, the problem came up again.

I tried moving the panel to other edges of the screen, trying to work around the bug. Now, it got stuck in an infinite loop, flickering at the bottom right corner, and causing X to use 90% of the CPU. The whole desktop is unusable now. Restarting plasma doesn't help.

Here's a sample stack trace from plasma-desktop:

#0  0x00007f56ddf29f83 in *__GI___poll (fds=<value optimized out>, nfds=<value optimized out>, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:87
#1  0x00007f56d3fe829a in ?? () from /usr/lib/libxcb.so.1
#2  0x00007f56d3fe9c2c in xcb_wait_for_reply () from /usr/lib/libxcb.so.1
#3  0x00007f56dcaba8d3 in _XReply (dpy=0x16c58b0, rep=0x7fffbba28ad0, extra=0, discard=0) at ../../src/xcb_io.c:461
#4  0x00007f56dca97697 in XGetWindowProperty (dpy=0x16c58b0, w=<value optimized out>, property=301, offset=0, length=2048, delete=0, req_type=4, 
    actual_type=0x7fffbba28c20, actual_format=0x7fffbba28c2c, nitems=0x7fffbba28c18, bytesafter=0x7fffbba28c10, prop=0x7fffbba28c08)
    at ../../src/GetProp.c:64
#5  0x00007f56dc380b24 in NETWinInfo::update (this=0x7fffbba28c70, dirty_props=<value optimized out>) at ../../kdeui/windowmanagement/netwm.cpp:3951
#6  0x00007f56dc3765da in KWindowSystem::setState (win=29365987, state=1024) at ../../kdeui/windowmanagement/kwindowsystem_x11.cpp:712
#7  0x00007f56de219dd0 in PanelView::checkShadow (this=0x2302340) at ../../../../plasma/desktop/shell/panelview.cpp:473
#8  0x00007f56de21d6f1 in PanelView::qt_metacall (this=0x2302340, _c=QMetaObject::InvokeMetaMethod, _id=<value optimized out>, _a=0x399b6f0)
    at ./panelview.moc:143
#9  0x00007f56db59387e in QObject::event (this=0x2302340, e=0x7fffbba28940) at kernel/qobject.cpp:1219
#10 0x00007f56da72ad2f in QWidget::event (this=0x2302340, event=0x399b650) at kernel/qwidget.cpp:8616
#11 0x00007f56daaefb06 in QFrame::event (this=0x2302340, e=0x399b650) at widgets/qframe.cpp:557
#12 0x00007f56dab7d32b in QAbstractScrollArea::event (this=0x2302340, e=0x399b650) at widgets/qabstractscrollarea.cpp:996
#13 0x00007f56de219a80 in PanelView::event (this=0x2302340, event=0x399b650) at ../../../../plasma/desktop/shell/panelview.cpp:1507
#14 0x00007f56da6d5cdc in QApplicationPrivate::notify_helper (this=0x16c06f0, receiver=0x2302340, e=0x399b650) at kernel/qapplication.cpp:4389
#15 0x00007f56da6db7cd in QApplication::notify (this=0x16a4a40, receiver=0x2302340, e=0x399b650) at kernel/qapplication.cpp:4270
#16 0x00007f56dc237eb6 in KApplication::notify (this=0x16a4a40, receiver=0x2302340, event=0x399b650) at ../../kdeui/kernel/kapplication.cpp:310
#17 0x00007f56db581a8c in QCoreApplication::notifyInternal (this=0x16a4a40, receiver=0x2302340, event=0x399b650) at kernel/qcoreapplication.cpp:732
#18 0x00007f56db5849b2 in QCoreApplication::sendEvent (receiver=0x0, event_type=<value optimized out>, data=0x1689080)
    at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:215
#19 QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=<value optimized out>, data=0x1689080) at kernel/qcoreapplication.cpp:1373
#20 0x00007f56db5ae043 in QCoreApplication::sendPostedEvents (s=<value optimized out>)
    at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:220
#21 postEventSourceDispatch (s=<value optimized out>) at kernel/qeventdispatcher_glib.cpp:276
#22 0x00007f56d2aa98c2 in g_main_dispatch (context=0x16c2cc0) at /build/buildd/glib2.0-2.24.1/glib/gmain.c:1960
#23 IA__g_main_context_dispatch (context=0x16c2cc0) at /build/buildd/glib2.0-2.24.1/glib/gmain.c:2513
#24 0x00007f56d2aad748 in g_main_context_iterate (context=0x16c2cc0, block=<value optimized out>, dispatch=<value optimized out>, 
    self=<value optimized out>) at /build/buildd/glib2.0-2.24.1/glib/gmain.c:2591
#25 0x00007f56d2aad8fc in IA__g_main_context_iteration (context=0x16c2cc0, may_block=1) at /build/buildd/glib2.0-2.24.1/glib/gmain.c:2654
#26 0x00007f56db5adb83 in QEventDispatcherGlib::processEvents (this=0x1688790, flags=<value optimized out>) at kernel/qeventdispatcher_glib.cpp:412
#27 0x00007f56da7877ee in QGuiEventDispatcherGlib::processEvents (this=0x7fffbba28940, flags=<value optimized out>)
    at kernel/qguieventdispatcher_glib.cpp:204
#28 0x00007f56db5807b2 in QEventLoop::processEvents (this=<value optimized out>, flags=DWARF-2 expression error: DW_OP_reg operations must be used eithr alone or in conjuction with DW_OP_piece.
) at kernel/qeventloop.cpp:149
#29 0x00007f56db580b9c in QEventLoop::exec (this=0x7fffbba29b70, flags=DWARF-2 expression error: DW_OP_reg operations must be used either alone or in cnjuction with DW_OP_piece.
) at kernel/qeventloop.cpp:201
#30 0x00007f56db584c4b in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1009
#31 0x00007f56de20fd06 in kdemain (argc=<value optimized out>, argv=<value optimized out>) at ../../../../plasma/desktop/shell/main.cpp:118
#32 0x00007f56dde6ec4d in __libc_start_main (main=<value optimized out>, argc=<value optimized out>, ubp_av=<value optimized out>, 
    init=<value optimized out>, fini=<value optimized out>, rtld_fini=<value optimized out>, stack_end=0x7fffbba29f38) at libc-start.c:226
#33 0x00000000004006c9 in _start ()

(That's what I see any time I interrupt it in GDB.)

(In reply to comment #4)
> I deleted all ~/.kde/share/config/plasma* files and restarted KDE. The panel
> got the correct width; however, as soon as I tried resizing it, the problem
> came up again.

Are you able to recompile KDE 4.5.x? If yes can you try to recompile kdebase-workspace with the change I suggest here: https://bugs.kde.org/show_bug.cgi?id=249948#c2 and see if it solves the problem?

I think that recompiling all kde is a little too big task for me. :)
I'll check if I can use the Suse src.rpm package that includes workspace.

I am still having this problem in KDE SC 4.6 Beta 2.

This is still a problem in 4.6 RC 1.

I can confirm this is an ongoing problem in 4.6.0.  It's most obvious with the panel resizing, but there may be other ways it shows up, e.g. when trying to full screen a flash video it only goes to the size of the smaller screen.

I also confirm the flash fullscreen problem. They might be related.

I still can confirm this bug on in 4.6.2. Maximum panel width is
restricted to width of smallest monitor. I have a TwinView setup (2 Monitors) with nvidia driver. However, on my system, this only affects panel on the bottom of the larger screen. Placing the panel on the top of the screen works perfect,
the maximum size will be correct.

In 4.8.3, I cannot reproduce this anymore. (Nor the full screen video issue, but that's not this bug per se). Can anyone here reproduce on a current version of KDE SC?

I'm away from my office, where I have different screen sizes. I'll return in two weeks. So, I'll keep this at my TODO. As soon as I return, I'll update this bug.