Please describe the problem:
When compose a new message, type something in "To" and "Subject" field, then
Press <Ctrl + Z> or Click Edit-> Undo. Undo function for these two fields do not
work. While it is works in body field. So when user first input some words in
body field and type Email address in the "To" field , at this time, use undo
function will change something in body field. This is not the user's intent. 

Steps to reproduce:
1. Compose a new message;
2. Type something in Body filed;
3. Type something in To or Subject field;
4. <Ctrl + Z> or Edit-> Undo;


Actual results:
Undo function happened in Body filed, while user want to undo in To or Subject
field. Unexpected changes happened in body fields.

Expected results:
User can undo in To or Subject field.

Does this happen every time?
yes

Other information: