Agree it's not so obvious how it works, but I have discovered it seems to be as follows.

You start typing and do not make any errors until the last word where you make a typo on last letter. eg: The owl and the pussy cat went to see

Then you press CTRL+Z to undo expecting it to undo the last thing you typed - letter 'e'. However, it will remove everything because it undoes anything from when you started the typing.

If you had started typing as before: eg: The owl and the pussy cat went to see Then used 'backspace' to remove the 'e' and then typed 'a' and then used CTRL+Z , you would find only the 'a' is removed because that letter 'a' is the start from when you commenced typing again. You had reset the cursor starting point.

Another example: Thus type eg: The owl and the pussycat went to sea Then move cursor (either by using mouse or arrow key) between y and c and then hit space and then used CTRL+Z , it would only remove the space you inserted. The 'Space' was the first movement forward of the cursor after positioning, so there was only a space to remove.

I have found that if you start typing and keep the cursor moving in a forward direction, a CTRL+Z will remove everything back to a point where text cursor last started moving in a forward direction. So if you moved cursor backwards at some point and commenced typing whether you used backspace key or manually used mouse to move text cursor or used backward arrow keys to move to a point within text to make correction, CTRL+Z goes to that point. If you never used any of those keys, so typing/ use of cursor was always in a forward direction, then a CTRL+Z will remove everything back to the point of when the last forward commenced which might be the start of a paragraph or even entire email.

So 'Undo' means undo everything since last start of typing after positioning text cursor.

Does this info help?

That helps a lot! Thank you so much!

It's kind of strange that Thunderbird's code works that way. I wonder if it can be improved upon. In the meantime I will just try to remember to hit Left Arrow, then Right Arrow, often.

That helps a lot! Thank you so much! It's kind of strange that Thunderbird's code works that way. I wonder if it can be improved upon. In the meantime I will just try to remember to hit Left Arrow, then Right Arrow, often.

It's not just moving cursor back and forth, you would need to use another key in between or using space will do the trick. Example: So type: the owl and the pussy cat Text cursor is now immediately after the 't' - Then press 'space' - then arrow backwards so cursor is again immediately after the 't' then hit space again and start to type again.

Or do after a full stop So type: the owl and the pussy cat. Text cursor is now immediately after the fullstop/period - Then press 'space' - then arrow backwards so cursor is now the otherside of the 'space' /after the fullstop, press space again and start typing. CTRL+Z will then go back to end of sentence.

If I have have completed a sentence, I press the full stop/period, then space (as you normally do), then back arrow or 'backspace' key, then press space again and continue typing, so a CTRL+Z will not remove a good sentence.

Using the 'backspace' key is best used if you only need to erase back one or two letters and less likely to cause anguish than a misused CTRL+Z.

Thank you again so much! I should have gotten that from what you first wrote. :-)

The devil is in the details. So few people seem to get that these days.