QPlainTextEdit corruption/crash after scrolling

1. Run this (I only tried on Windows):

QApplication a(argc, argv);

QPlainTextEdit* bugger = new QPlainTextEdit();
bugger->resize(160, 30);
bugger->document()->setPlainText("The quick bropwn fox jusmp over the lazy dog.");
bugger->show();

return a.exec();

2. Place text cursor right after "dog."

3. Drag with the mouse from that point, ending somewhere behind bottom right corner of the window,

4. The following can be observed:

• the text edit becomes irresponsive to keyboard navigation and mouse clicks
• it does not show text cursor
• does not repaint text if you minimize/restore the window
• pressing PageUp/PageDown in this state once caused Assertion failure & crash for me, however, I can not reproduce again.
  ASSERT: "n" in file ..\..\include\QtGui/private/../../../src/gui/text/qfragmentmap_p.h, line 291