I type the subject in compose window for new message and the variable %subject%
fill the content instantly  => the same like the TheBat! Mailclient

the question is how can typing the subject "fill the content instantly"? Once
you type the first letter in the subject and replace the %subject% tag it won't
know what and where to replace anymore.

There are 2 alternatives:

1. replace the %subject% before when the users clicks send. This would be the
safest way (as only then you can be sure that the subject has been completely
finalized). Disadvantage: not very intuitive for the users (they won't know
that it works until the hit the 'send' button)

2. replace the %subject% once the subject textbox looses focus.
Advantage: kind of intuitive as it give instant feedback
Disadvantage: this can only be done once, the "copied subject" in the email
text could not be updated later if the user edited it again (as the %subject%
variable would now be replaced). There might be some workarounds for this when
editing in HTML mode (e.g. a div class).

Personally I would favor (2) as it is less prone to errors and probably
reasonably simple to implement.

In reply to comment #1:
Maybe replace %subject% when the textbox loses focus, but undo that replace if
the textbox regains focus during the same session? Do you think it would be (a)
possible; (b) useful?

(Of course, also replace it if the message is sent by hitting Ctrl+Enter or
something while the textbox has focus.)

(In reply to comment #2)
> In reply to comment #1:
> Maybe replace %subject% when the textbox loses focus, but undo that replace if
> the textbox regains focus during the same session? Do you think it would be (a)
> possible; 

this is difficult, probably not worth the effort
> (b) useful?

anything that saves the user work can be considered useful :)
> 
> (Of course, also replace it if the message is sent by hitting Ctrl+Enter or
> something while the textbox has focus.)

Also replace on send, I agree. I think this is definitely useful; the difficult
thing is to make sure users do not delete it or think it is a bug if they do
not write anything in the subject line.

This one could open up a lot more useful features once the problem of "late
resolving" of variables is cracked. Basically ST4 does its processing when
Composer opens, but for this one we would need to resolve the variable
(%subject%) to a temporary placeholder tag (e.g. <span>) 

<span id="SmartTemplate4_subject">%subject%</span>

and then have an event  (e.g. subject textbox losing focus) which would update
its content. The tag would also have to be removed on send in order to
"finalize" the operation and avoid accidental replacement on subsequent
replies.

We could also add a "Update fields"button  (like MS word had it) maybe?

I am adding support for "late resolving" of missing headers with the new
clickable ST4 fields feature. These variables will be shown with a colored
(pink) placeholder that can be clicked during composition of the emails.

Created an attachment (id=8532) [details]
1.6 prerelease 139

With this version, the following functions can now also be used in "New"
Emails:
%subject%
%to%
%from%
%cc%
%bcc%

these will be pulled from the address widget / subject line when the colored
field is clicked on. Clicking can be done multiple times so subject line can
still be changed and updated.

A separate button for the composer toolbar is provided to "clean up" all fields
before sending - this means any special code that makes these variables update
/ look different is removed and the content is finally inserted as "normal
text".

Also supports the new %dateformat% from [Bug 26588]

Created an attachment (id=8534) [details]
1.6 prerelease 168

Added support from the variables tab and slightly better styling behavior. 

Variables tab
Added a new color / behavior for the variables that can now be added to the
"New" template. These (orange) boxes can now be added just like the green ones
without triggering the Warning "the variable xxx cannot be user for new
messages"

Styling
The orange / pink field styles are now loaded only when the Composer is opened.
(Unfortunately it still affects the main window as well at this point in time)

Links to Bugs
links to Bug descriptions in the Variables tab now open a new browser tab in
Thunderbird

Created an attachment (id=8546) [details]
1.6 prerelease 244

After some more testing, I also support this type of "Late variable resolution"
on address fields in forwarded emails. In order to distinguish it from the
original headers, you need to add the parameter "fwd" in the address
parameters. Example:

%to(firstname,fwd)% - will fill in the firstname of the Addressee field once
clicked
%from(email,fwd)% - fills in the email address of the sent from field in the
address widget
%cc(name,??bracketMail(),fwd)% - fills in the name or (if name cannot be
determined) <email address> of somebody the forwarded to as a cc.

- without the fwd parameter, the address parameters all refer to the headers of
the forwarded email as before.

Implemented in Release 2.0 (23/12/2018)