Until today, on latest Chrome browser, Gmail Compose/Reply window offered a button at top right to pop-out to a separate tab.
Now, it is not there.
Where is it now? If anywhere....
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Sign up to join this communitySHIFT-click the Compose/Reply window's (mis-named) Full Screen button:
Note: SHIFT hover changes the button to a tooltip-less single arrow:
SHIFT
when clicking on the Exit full screen icon e.g. image in the top right corner of the partially popped out window.