I think the clue is the wording of the text in Mirko's first screenshot - it is a continuous line of the word "Help" with no spaces between the words. Previewing what is essentially one very long and solid line of characters, seems to mess up the forum software.
I can replicate the problem by doing the following:
Prepare a similar message (one continuous chunk of characters without a space between them) which takes up about 3 lines of the Post Reply message preparation pane:
Click the preview button
Since there are no breaks, the preview pane does not know how to split the long line of text into different lines, so it tries to display it on one line.
In order to display it on one line, the software creates a horizontal scrollbar in the preview window so that you can scroll from left to right along the line:
Once this horizontal scrollbar has been created, the preview window will remain narrow top to bottom, even if the continuous line of text is deleted and replaced by normal next with spaces and line breaks:
One way to reset things to normal is to refresh the window in your browser (press F5 in Internet Explorer on a Windows machine) which will delete all your text and allow you to start from scratch. Another method is to exit from the forum and come back again.
In summary, when the user creates a long line of characters which necessitates the creation of a horizontal scrollbar in the preview pane, a bug in the forum software fixes the vertical size of the preview window for the remainder of that message creation session.
Forrest