This was first mentioned in a post about blank WPide screens having no file manager.
When in Opera, the following happens: try to type for instance "echo('hello world')", the context-sensitive helper popup for "echo()" pops up in the top-right of the editor window and stays there, even when you close and reopen the file, or open another file, hiding the part you were actually typing.
When in Firefox, whenever you type a character, all characters typed so far are also echoed in the editor. So, if you type "ABC" you see "ABCDABCABA" in the editor. When you save it, the text "ABCDABCABA" will be saved. Note also, that the blinking cursor remains on the spot where you first started typing (in the example before the first "A") but the characters are inserted at the end of the string that you are inserting.
Also, in Firefox, the context sensitive popup is about one millimeter wide, has a scroll-bar, and doesn't disappear anymore, not even when you open another file.
In Internet Explorer and in Chrome, it seems that both the context-sensitive popups and the typing works normally.