Bugzilla

> Bob says Note: The ability to Preview a suspect or any email without opening it was the single best feature in Thunderbird email. 

This never existed. You seem to be completely misguided and believing something for a long time which was not true. 

AS Magnus says:
> Earlier the document was also opened, only in the background....There wouldn't have been any security advantages if that's what you were after.

What does that mean: Previously, if you right clicked on new unopened email and select 'Print Preview' - the email document was opened in the background in order to read the contents, so they could be shown in the 'Print Preview'. 
The 'Print Preview' means seeing what you are about to print before you print it.  
It does not mean get a look at something without opening the document to see what it contains.  If you have been believing that was the case then you were mistaken. The only difference was the email in the list was not marked as read and the email document was opened in the background in order to extract contents.

> Bob says - the message pane which displays the contents of the email,  it then marks the email as Read.  That is the same as just opening an email which is NOT what a correct Preview should do. 

There is no such thing as a preview of an email without opening the email document to see contents.  This has never existed.  This has already been explained several times. 

Currently, 'Print' is enabled if the Mark as Read option is selected either automatically or manually, but that does not mean the email is open, so the result is the preview of a Print is blank - it is blank because you have not selected to open the email and the new code means it is no longer opened; without  your knowledge; in the background.

IF you want to open email, but do not want any automatic option to mark as read, then switch off that option so that you have full manual control over the 'Mark as Read'. You do that in the Preferences.
Menu app icon > Preferences > General - scroll down to the 'Reading & Display' section to see options.

After switching off the auto mark as read in Preferences:
Select email so it shows in the Message Pane
Right click and select 'Print' to see what you get when you print the email. 

However, I cannot make this more clear -  there is no need to use 'Print' to see a Print Preview just to read email contents and there never was any reason to open an email in Print Preview if no intention to Print. 

Bob, you have helped to locate this bug which is good and helpful. We are very grateful to you bring this error to our attention.

> Bob says Note: The ability to Preview a suspect or any email without opening it was the single best feature in Thunderbird email. 

This never existed. You seem to be completely misguided and believing something for a long time which was not true. 

AS Magnus says:
> Earlier the document was also opened, only in the background....There wouldn't have been any security advantages if that's what you were after.

What does that mean: Previously, if you right clicked on new unopened email and select 'Print Preview' - the email document was opened in the background in order to read the contents, so they could be shown in the 'Print Preview'. 
The 'Print Preview' means seeing what you are about to print before you print it.  
It does not mean get a look at something without opening the document to see what it contains.  If you have been believing that was the case then you were mistaken. The only difference was the email in the list was not marked as read and the email document was opened in the background in order to extract contents.

> Bob says - the message pane which displays the contents of the email,  it then marks the email as Read.  That is the same as just opening an email which is NOT what a correct Preview should do. 

There is no such thing as a preview of an email without opening the email document to see contents.  This has never existed.  This has already been explained several times. 

Currently, 'Print' is enabled if the Mark as Read option is selected either automatically or manually, but that does not mean the email is open, so the result is the preview of a Print is blank - it is blank because you have not selected to open the email and the new code means it is no longer opened; without  your knowledge; in the background.

IF you want to open email, but do not want any automatic option to mark as read, then switch off that option so that you have full manual control over the 'Mark as Read'. You do that in the Preferences.
Menu app icon > Preferences > General - scroll down to the 'Reading & Display' section to see options.

After switching off the auto mark as read in Preferences:
Select email so it shows in the Message Pane
Right click and select 'Print' to see what you get when you print the email. 

However, I cannot make this more clear -  there is no need to use 'Print' to see a Print Preview just to read email contents and there never was any reason to open an email in Print Preview if no intention to Print. 

Bob, you have helped to locate this bug which is good and helpful. We are very grateful to you bring this error to our attention.
The 'print' needs to be disabled if email is not open.