if you double click to maximize the viewer you can't reattach it back. 

Reproducible: Always

Steps to Reproduce:
1. double click on viewer to maximize
2. try to reattach it again
3.



(gdb) run
Starting program: /usr/bin/kdenlive 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
kf5.kxmlgui: Registering action  "edit_select_all"  under new name  "select_all_tracks"
Removing cache at "/home/farid/.cache/kdenlive-thumbs.kcache"
0x55555737bbc0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x5555561c08e0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557964dd0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557a054a0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557bbaef0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x55555737b780 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x55555737af10 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557930520 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x5555578f01d0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x55555795ab30 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557951860 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557953740 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x5555579532b0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557363110 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557362cd0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x55555710e9d0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x5555574dacc0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557101400 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x5555574dbbb0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x5555574de0f0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x5555570fde30 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557378070 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557c035d0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557c00a60 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557c1cf60 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557c4dd10 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557c309d0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x555557b48cd0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x55555737bbc0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557964dd0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557a054a0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557bbaef0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x55555737b780 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x55555737af10 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557930520 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x5555578f01d0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x55555795ab30 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557951860 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557953740 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x5555579532b0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557363110 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557362cd0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x55555710e9d0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x5555574dacc0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557101400 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x5555574dbbb0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x5555574de0f0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x5555570fde30 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557378070 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557c035d0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557c00a60 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557c1cf60 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557c4dd10 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557c309d0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x555557b48cd0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
Removing cache at "/home/farid/.cache/kdenlive-thumbs.kcache"
// TRACTOR PROBLEM
[New Thread 0x7fffaffff700 (LWP 4821)]
[New Thread 0x7ffeabff6700 (LWP 4825)]
[Thread 0x7ffeabff6700 (LWP 4825) exited]
[Thread 0x7fffaffff700 (LWP 4821) exited]
[mp3 @ 0x7fffa940c8a0] Header missing
0x55555762e860 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x55555762e860 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x55555762eeb0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x55555762e860 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x55555762e860 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x55555762e860 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
0x55555762e860 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x55555762e860 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eb1c50) ): Attempt to set a screen on a child window.
QXcbConnection: XCB error: 8 (BadMatch), sequence: 64682, resource id: 23068706, major code: 1 (CreateWindow), minor code: 0
QXcbConnection: XCB error: 3 (BadWindow), sequence: 64685, resource id: 23296560, major code: 18 (ChangeProperty), minor code: 0
0x55555762e860 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
0x5555579557e0 void QWindowPrivate::setTopLevelScreen(QScreen*, bool) ( QScreen(0x555555eab710) ): Attempt to set a screen on a child window.
Couldn't start kded5 from org.kde.kded5.service: QDBusError("org.freedesktop.DBus.Error.ServiceUnknown", "The name org.kde.kded5 was not provided by any .service files") , falling back to running kbuildsycoca5
kf5.kservice.sycoca: Trying to open ksycoca from "/home/farid/.cache/ksycoca5"
Calling notify on "Sound"
Audio notification requested, but sound file from notifyrc file was not found, aborting audio notification
Calling notify on "Taskbar"
0 23359128
Closing notification 1
not found dbus id to close 1
QXcbConnection: XCB error: 3 (BadWindow), sequence: 23262, resource id: 23296560, major code: 4 (DestroyWindow), minor code: 0
[Thread 0x7ffff7f4e7c0 (LWP 4658) exited]
[Inferior 1 (process 4658) exited normally]
(gdb) backtrace