This is an old problem, still not solved. If one opens an event in a separate window, the window only has a close button, but no minimize/maximize buttons.
I used to install an older version of lightning manually after each update of Thunderbird (as the TB update installed the latest version of lightning).
That older version had some config files being present uncompressed within lightnings subfolder, which allowed to make the event window minimizable. This setting was activated by default for other operating systems. I cannot see any reason why it was deactivated by default for Windows.
Anyway, with the older version of lightning I could activate it manually.
But that older version of lightning is not compatible with the latest version of Thunderbird. I know one could fiddle around with the app.ini within the lightning addon .xpi container, but I do not want to risk "destroying" my calendar data. ;)
Why can't the possibility to maximize/minimize event windows not be activated by default for Windows as it is for other OSs? Is there a particular reason for that?
I can't use the new option to display events in a tab within the main application window. That option bears some problem which I mentioned in another post: Ctrl+S does not work for saving the event, one would have to click the "Save & Close" button in order to save but then of course the event will be closed. I use events to maintain a text list in the description box which I amend several times during the day, so I just like to keep it open.
This would not so much be a problem if there wasn't yet another problem, which I also stated in another post: When closing Thunderbird and there are unsaved changes in an event window (no matter whether displayed in a separate window or in a tab), this is recognized and the software asks you whether you want to save the changes, discard them or cancel closing Thunderbird. But choosing "Save" does not save the changes. Thunderbird closes and the changes are lost.
When the event is displayed in a separate windows, at least Ctrl+S works, so my changes during the day were saved, only the last changes which I did not save with Ctrl+S were then not saved upon closing Thunderbird.
But with an event opened in a tab, Ctrl+S never works, so in the end when Thunderbird is closed and despite choosing to save the changes when Thunderbird asks, nothing was saved, so on re-opening TB and that even, the description box was completely empty.