Regression was introduced in d0e854e9d8.
Each time you hide() a widget its mousePress state is reset and
mouseMoveEvent() / mouseReleaseEvent() handlers are never called.
So you can't toggle (hide + show) widgets that are mouse-pressed.
Fixes#4802.
Currently the build without implicitly included precompiled header
is not supported anyway (because Qt MOC source files do not include
stdafx.h, they include plain headers).
So when we decide to support building without implicitly included
precompiled headers we'll have to fix all the headers anyway.