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bug#22295: viper-mode undo bug introduced between Nov 10 and Nov 14

From: Jim Meyering
Subject: bug#22295: viper-mode undo bug introduced between Nov 10 and Nov 14
Date: Mon, 16 May 2016 08:39:15 -0700

On Mon, May 16, 2016 at 5:41 AM, Phillip Lord
<address@hidden> wrote:
> I have pushed a first attempt at a fix to branch fix/viper-undo.
> Clearly, it's not finished yet, but I'd like to get your feedback as to
> whether it works; it should fix the example given, but I haven't used
> viper at all, so if it breaks other things I won't know.
> I haven't managed to work out exactly why the error is happening --
> probably though because the timing of the undo-boundary is somewhat
> different, though. With this fix, viper just disables automatic boundary
> addition and adds it's own as necessary, which seems cleaner.

Thank you.
That does indeed fix the case I mentioned.
Here are some cases where it does not work as expected:

  start with an empty buffer in viper-mode
  type 'i 1 2 3 4 5ESC'
  type 'F2dw' to delete the 2 and a space.
  type 'wdw' to delete the 4 and a space.

Now, if I were to hit "u" to undo, I would expect that most recent
deletion to be undone and the 4 would reappear.
Then I would hit '.' to undo the deletion of the '2'. Finally one more
'.' would undo the creation of that first line.
However, with the current patches, that first 'u' undoes everything
and leaves me with the empty initial file.

Another example starting with an empty file:
Create some content via ':r!seq 999|fmt RETURN'
Then remove e.g., "222 " and "444 " via '/222' RET 'dw',
then '/444' RET 'dw'. Now, we expect a single 'u' to restore the '444 ',
yet it undoes everything, leaving an empty buffer.
Hmm... that's probably no different from the first example.

One more, then. Starting with this input:

  1 2 3 4 5 6

advance to the '2' with 'w', 'dw' to delete the 2, then three '.'s to
delete the 3, then 4 and 5.
Then begin to undo with 'u', then '.' to repeat it. Those first two
work, restoring the 5 and 4.
However, one more '.' restores both the 3 and the 2.

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