* Fix an issue with the canvas editor when the canvas is embedded

Fix a problem with the editor not appearing when the TCanvas is embedded, [as reported on the Forum](https://root-forum.cern.ch/t/editor-not-appearing-in-gui-canvas/53968

)

Co-authored-by: Axel Naumann <Axel.Naumann@cern.ch>

* Cosmetics (thanks Axel!)

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Co-authored-by: Axel Naumann <Axel.Naumann@cern.ch>