Tried twice, think is consistently behaving:

• open Page1
• open Editor on Page 1
• leave Editor window open
• change to another Page2 in Preview, using actual links
• in Customizer, Back
• now both pages are listed
• click on the other Page2
• Editor window still open, with text from Page1
• make any edit, Save
• Content from Page1 saves to Page2
  Not good :)