Check whether scroll is finished using AppleScript - scroll

Question

I have an AppleScript script to open the Mail app, open a mail, scroll down and take screenshot of the whole mail body. I am taking the screenshot by scrolling the page using keystroke 121 (page down). I don't know how much time I have to repeat the loop to reach the page end. Is there anyway we can identify if the scroll has reached the page end using AppleScript?
This is my code snippet for page down and taking screenshot:
open each_message
tell application "System Events"
delay 5
keystroke "f" using {command down, control down}
delay 3
key code 20 using {command down, shift down}
delay 5
key code 121
delay 5
key code 20 using {command down, shift down}
delay 5
keystroke "f" using {command down, control down}
end tell

Answer 1

After reading your question and ensuing comments...
Is there anyway we can identify if the scroll has reached the page end using AppleScript?
Yes
For some emails, it's fine. For some, it is duplicating the last page, for some, the screenshots are incomplete.   (From comments under the question.)
That is the nature of different length documents and pressing the Page Down key, and why maybe using a third-party application like Snagit is a better way to go.
That all said...
The example AppleScript code, shown below, was tested in Script Editor under macOS Catalina with Language & Region settings in System Preferences set to English (US) — Primary and worked for me without issue1.
1 Assumes necessary and appropriate settings in System Preferences > Security & Privacy > Privacy have been set/addressed as needed.
As you did not include the complete code of your script, the following example AppleScript code is presented more as a proof of concept. It too may/will have extra screenshots, however, it ensures the page it all the way to the top before it starts taking the screenshots and ensures a screenshot of the whole last page is taken too, based on it stopping appropriately.
For testing purposes I started with an individual email opened in its own window, assuming that is what the open each_message at the top of the code you included does or represents.
Example AppleScript code:
tell application "Mail" to activate
delay 1
tell application "System Events"
keystroke "f" using {command down, control down}
delay 1
repeat while my getEmailFullScreenWindowScrollPosition() > 0.0
key code 116
delay 0.1
end repeat
delay 1
repeat while my getEmailFullScreenWindowScrollPosition() < 1.0
key code 20 using {command down, shift down}
delay 1
key code 121
delay 1
end repeat
key code 121
delay 1
key code 20 using {command down, shift down}
delay 1
keystroke "f" using {command down, control down}
end tell
to getEmailFullScreenWindowScrollPosition()
tell application "System Events" to ¬
return value of ¬
scroll bar 1 of ¬
scroll area 1 of ¬
window 1 of ¬
process "Mail"
end getEmailFullScreenWindowScrollPosition
Notes:
In Mail in macOS Catalina on my system there was no horizontal scroll bar when an opened email in its own window is in Full Screen view, and why the handler is directed to scroll bar 1. However, if you do need to query for its orientation, then for the handler use:
to getEmailFullScreenWindowScrollPosition()
tell application "System Events" to ¬
return value of ¬
first scroll bar of ¬
scroll area 1 of ¬
window 1 of ¬
process "Mail" whose ¬
orientation is ¬
"AXVerticalOrientation"
end getEmailFullScreenWindowScrollPosition
In testing the getEmailFullScreenWindowScrollPosition() handler, it would report 1.0 when there was still some page left but not showing on the screen, so I added the additional lines of code after the second repeat loop to ensure it was all captured.
The test email had content to the very end of the page and maybe isn't really representative of most emails. If you find that there is ample white space at the end of your emails then you may want to remove the following lines of code after the second repeat loop:
key code 121
delay 1
key code 20 using {command down, shift down}
Also note that I ended up using delay 1 for all but one of the delay commands, and it worked fine for me in testing, however, adjust as necessary for your system.
As this is just being presented as a proof of concept, I'll leave it to you to tweak the code to fit your needs.
Note that I am not affiliated with the developer of Snagit and have mentioned it as a possible option to capturing the entire scroll area of the screen in a single screenshot.
Note: The example AppleScript code is just that and sans any included error handling does not contain any additional error handling as may be appropriate. The onus is upon the user to add any error handling as may be appropriate, needed or wanted. Have a look at the try statement and error statement in the AppleScript Language Guide. See also, Working with Errors. Additionally, the use of the delay command may be necessary between events where appropriate, e.g. delay 0.5, with the value of the delay set appropriately.

Answer 2

Here's a handler that returns the current vertical scroll bar position for a message viewed in the frontmost message viewer. It makes a bunch of assumptions about which panels you have open and what orientation the viewer has, but it should give you something to work with:
set sp to my findMailScrollPosition()
on findMailScrollPosition()
tell application "System Events"
tell process "Mail"
tell window 1
tell splitter group 1
tell splitter group 1
tell scroll area 2
tell (first scroll bar whose orientation is "AXVerticalOrientation")
-- this will return a value between 0 and 1 (top to bottom)
return value
end tell
end tell
end tell
end tell
end tell
end tell
end tell
end findMailScrollPosition