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alansplace - You can show and hide the toolbar with the Ctrl/F11 toggle - just like you can in the old viewer. The difference is that old viewer's default setting is SHOW, whilst the new viewer's default setting is HIDE.
In the old viewer there's also an option to hide/show the toolbar in the context (right click) menu, not sure if there's anything similar in the new viewers right click options matrix - I get lost in it
Another new feature is that new viewer's toolbar can be customised much the same as the calibre library manager's toolbar can be.
Here you can see the viewer's default toolbar:
Added: I don't have any clues as to why windows go off screen on some peoples computers - I suspect it's something in Qt.
BR