bug#21884: 25.0.50; Unexpected indentation from `open-line' due to new `electric-indent-mode' default.

[Karl Fogel]

Note that this bug report summarizes a discussion on the Emacs Devel mailing 
list -- please check there for updates:

  "Questioning the new behavior of `open-line'."
  https://lists.gnu.org/archive/html/emacs-devel/2015-11/threads.html#01100

The problem:

Since Artur Malabarba's commits of 2015-10-24 in simple.el, `open-line' has a 
new indentation behavior, because those commits made `open-line' sensitive to 
`electric-indent-mode', which in turn is now on by default.

For example, suppose you have the following text in a buffer, in just about any 
mode (i.e., not necessarily a programming language mode), and indented by two 
spaces as below:

  A lovely haiku
  An example for this bug
  For Emacs Devel

Now place point where the imaginary "X" is below and do C-o (`open-line'):

X A lovely haiku
  An example for this bug
  For Emacs Devel

Formerly, that would literally insert a newline, but not remove the the
two spaces of indendation that come *after* the new newline (the two
spaces right before "A lovely haiku").  So this would be the result:

[...this is the new line; imagine it's blank...]
  A lovely haiku
  An example for this bug
  For Emacs Devel

But after the recent commits, those two spaces get removed:

[...this is the new line; imagine it's blank...]
A lovely haiku
  An example for this bug
  For Emacs Devel

Thus one can no longer just put point in column 0 and type C-o to open a line 
right before a block of indented text while preserving the block's indentation.

Solutions:

One solution would be to revert `open-line' to being entirely insensitive to 
`electric-indent-mode'.  But Artur points out (in 
https://lists.gnu.org/archive/html/emacs-devel/2015-11/msg01100.html) that when 
point is in the middle of a line, one might want electric-indent behavior from 
`open-line'.  For instance, in the above example, put point at the first ‘h’ 
and do C-o.  If `open-line' just ignores electric-indent, then this would be 
the result:

  A lovely
haiku
  An example for this bug
  For Emacs Devel

Whereas with Artur's changes, the result would be this (which at least some 
people, including Artur, would desire):

  A lovely
  haiku
  An example for this bug
  For Emacs Devel

John Wiegley has proposed that `open-line' just be aware of when point is in 
column 0 and not do the electric-indent behavior then.  This preserves the 
common use case of C-a C-o, but also preserves Artur's "fix" (for those who 
agree it is a fix) for when C-o is invoked in the middle of a line.

Discussion is ongoing as of this writing.