Minimized windows shown as small rectangles

[midi-pascal]

Qt 4.8 and 5.4, Qt Creator 3.1 and 3.3.0 under Ubuntu 12.04

Since installation of Qt 5.4 and Qt Creator 3.3.0 my Qt apps show a defect.

If I minimize a subwindow of one MDI app, its corresponding "tab" that was showing previously at the bottom left of the main window with its icon and window tittle now shows as a tiny rectangle (I assume it is the tab few pixels wide):

Minimized.jpg

In this example there are 3 minimized subwindows so 3 tiny rectangles.
Even if I rebuild (qmake, clean and rebuild) with Qt Creator 3.1.2 using Qt 4.8 the defect remains.
I did not modified the code since the Qt/Qt Creator upgrade, only recompiled the app.
The small rectangle (tab) is active since the window menu can still be used by hovering the mouse curson on it and I can restore the window with it.

Any help on this would be welcome!