Commit ac2818 affects the refresh performance on Windows

In our code editing software (https://github.com/aseba-community/aseba), the editor is inherited from QTextEdit. The GUI is single threaded.

With Qt 4.8, the widget's refresh is considerably slowed down on Windows when typing new characters (tested on Windows XP and Seven 64-bits). This was not experienced with Qt 4.7.4.

After bisecting, the problem appears with the following commit:

ac2818bef95f1347e80be8ce847f2557339b76e0
Trond Kjernåsen <trond@trolltech.com> 2010-04-07 14:57:24
Make it possible to call QWidget::metric() from a different thread.
Under Windows.

Reverting the commit in Qt 4.8 solves the issue.