bug#31104: 26.1; Undo limits

[Nick Helm]

I keep bumping into Emacs's undo limits. 

The recipe below uses an org-table to illustrate the problem. That's
where I encounter it most often, but any command that changes a large
amount of text will do.

Emacs -Q
;make a tall org-table
  M-x org-mode
  "|Column One|Column Two|Column Three|" RET ;enter a table row
  C-p C-k C-k
  C-x ( C-y C-x )
  C-u 1000 C-x e
;delete a couple of columns
  C-a
  M-S-<left> ;delete Column One
  M-S-<left> ;delete Column Two
;undo both deletes
  C-/ ;Column Two reappears
  C-/ ; -> "No further undo information"

In this scenario, Emacs provides just one undo step and Column One is
lost for good. All previous undo information is also silently lost. This
can be troublesome if work is not saved. I expect Emacs to always
remember at least a handful my most recent changes if only to protect me
from bad keystrokes.

I know I can change the undo-*-limit variables to improve this
situation, but I don't think users should have to do this.

Bit of further navel-gazing...

Simply raising the default undo limits (or advising users to raise it in
their config) has problems too. Given a large enough edit, I've had the
same issue reappear. I find it difficult to work out how an undo limit
set in bytes translates into real-world editing behaviour.

Perhaps a byte size is not the best way to specify undo limits, at least
not for user-facing settings? Instead, could Emacs guarantee a minimum
number of undo steps, say the 20 most recent, regardless of the amount
of data it consumes? 



In GNU Emacs 26.1 (build 1, x86_64-apple-darwin17.5.0, NS appkit-1561.40 
Version 10.13.4 (Build 17E199))
 of 2018-04-06 built on oberon.local
Windowing system distributor 'Apple', version 10.3.1561
Recent messages:
Opening nnimap server on Office365...
Opening connection to localhost via shell...
Opening connection to localhost...done
Opening nnimap server on Office365...done
Opening nntp server on Gmane...done
No new newsgroups
Checking new news...
Reading active file via nnnil...done
Reading active file via nndraft...done
Checking new news...done

Configured using:
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