Scrollbars in QPlainTextEdit broken during resize

The QPlainTextEdit's scrollbars behave wrong when the text contains wrapped lines. Observe 2 unwanted behaviours with the attached ui file (can reproduce it from QDesigner). Notice that first line contains very long text.

Strange behaviour 1:
1) Preview the attached form and scroll down to the last line using vertical scrollbar. Leave the text cursor in the first line.
2) Resize the window horizontally by dragging the left edge of the window (just about couple of pixels so that the first line is not rewrapped). When you finish resizing and release the mouse button the scrollbar dances.

Strange behaviour 2:
1) Preview the attached form and scroll down to the last line using vertical scrollbar. Move the text cursor to the last line this time.
2) Resize the window horizontally by dragging the left edge of the window (just about couple of pixels so that the first line is not rewrapped). When you finish resizing and release the mouse button the scrollbar handle changes its size.
3) Now click several times the up button of the vertical scrollbar. Notice, that when you see a line with text "1" and press the up button again, the view scrolls by several lines, instead of just one line.