[XCB] Incorrect cursor shape when moving mouse with pressed mouse button

Steps to reproduce using software from the attachment:

• move mouse cursor to the center of the window (cursor shape is now CrossCursor),
• click and hold left mouse button,
• move the mouse outside the window (the best way to reproduce is to move through the title bar) (now the cursor shape is still CrossCursor with Qt 5.5(OK) or ArrowCursor with Qt 5.4(BAD)) ,
• move (slowly) mouse cursor to the center of window (now the cursor shape is ArrowCursor in Qt 5.5 and Qt 5.4, but should be CrossCursor),
• release left mouse button - the cursor still remains ArrowCursor instead of CrossCursor.

When using Qt 5.5 if you decrease layout margin and you move the mouse quickly, CrossCursor remains.

Expected results: while mouse button is held, the cursor shape must not change (like in Qt4).

It affects all Qt5 applications including QtCreator under Linux/XCB.