Additional info on odd display which now sees wrong, non selected emails being Archived.
Settings > General
REading and Display
Select 'A new message window'
Do not select 'Close message window/tab on move or delete.
Enable the Message Pane
In list there are 4 emails, A, B, C, D where A is the top one.
Sorted by Date, Descending, unthreaded
Select 'B' to highlight it opens in Message Pane
then open email 'B' using double click so it also opens in a new window.
press 'A' key and email 'B' is Archived.
In Message List email 'C' is now highlighted as the email above has been removed and list is moved up to fill gap.
Email 'C' is displaying in Message Pane - this is correct.
Email 'A' is displaying in the open window. - this is incorrect.
Email 'C' should have remained open in window because I have disabled the 'close' on move option.
Email 'A' although incorrectly selected should not have opened at all because it was not a double click and if opened should be in a new window.
Leave window open displaying email 'A'.
In Message List Select email 'C' - even if already highlighted - select it - you now have focus on the main UI and not the email displying in open window .
Email 'C' it is already displaying in Message Pane
Email 'C' is not Archived and remains selected in Message List and remains displaying in Message Pane - this is unexpected.
However, Email 'A' - the non selected email that was displaying in the non-focused open window has been archived. This is not expected.
Email 'A' disappears from Message List because it got archived.
Email 'C' now displays in the open window.
It would seem enabling the 'Message Pane', also over rides the double click action and displays email in open window.
'Edit' > 'Undo move message' recovers the archived Email 'B' - it does not not undo the last erroneously archived email 'A'.
There is something very odd going on if you have an email already opened in a window and do not have the setting to auto close on move.