bug#21489: 25.0.50; Echoing and message interaction (Bug#21489)

[Paul Eggert]

I'm following up on <http://bugs.gnu.org/21489> dated 2015-09-15.

I tried the example code you gave, and it behaved oddly for me under 
Emacs 24.5. I ran the program and typed "ab" and Emacs echoed "Hello" in 
the minibuffer. I then typed "bbbbbbbbbbb" and nothing happened. I then 
typed "c" and Emacs echoed "a c is undefined" in the minibuffer. (I 
didn't do this because I knew what would happen; I was just trying 
things idly.)

In contrast under Emacs 25.0.50, when I typed "ab" Emacs delayed and 
then echoed "a-". Then, each time I typed "b" it cleared the minibuffer, 
delayed, and then echoed "a-". When I finally typed "c" it it echoed "a 
c is undefined". This behavior more clearly indicated to me what was 
going on (which I figured out only after a bit of thought).

This exercise was unrealistic enough that I expect we shouldn't be 
basing design decisions on it. That being said, for this particular use 
case the 25.0.50 behavior appears to be a minor improvement over 24.5.

I suspect you filed this bug report because of some other, 
more-realistic code that behaves poorly with 25.0.50's echoing behavior. 
If so, could you please explain what the code is and what the 
misbehavior's symptoms are?