Found a bug - not sure if caught and fixed already in upcoming release…
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Open the page CSS from the left sidebar and expand it to cover the whole page, then click X to close it.
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Open the Undo feature from the left sidebar. It will expand to cover the whole page, and since this control lacks the minimize function, you can’t narrow it to see the page you’re working on.
To fix, you must go back and open CSS and then minimize it. Then Undo will open in its normal width.