WYSIWYG zoom mode does not fit inside window in Internet Explorer - works in FF
Nice new feature!
But when you use it in IE the window is approx one scroll bar too wide so the browser adds a horizontal scroll bar and you cannot see the scrollbar of the zoom window.
To see this bug you must view a long topic that needs scrolling, in zoom mode, in IE.
The zoom tool is a Kupu feature, and the behaviour you describe is Kupu behaviour. It is not specific to the WysiwygPlugin
Note that you can play with "raw" kupu as follows:
This same file is in your install. It opens the editor on an example called "fulldoc.html" in the same dir.
You wouldn't have a reference to Kupu? Then I'll raise a bug report there.