more problems with line-move

[Luc Teirlinck]

The following problems are different from the ones discussed in the
related thread "invisible" in that they involve visible intangible.  I
mentioned such problems before (in October), but since we are
discussing line-move, it might be good to remember that these problems
exist.

Unlike invisible intangible, visible intangible seems to have no
problems with an intangible newline, as long as the character after
that newline is not intangible.  If it is intangible however, trouble
develops.


Visit the following file with emacs -q:

===File ~/lines=============================================
line1
line2
line3
line4
line5
line6
line7
============================================================

No trailing whitespace, but trailing newline.

M-: (put-text-property 10 18 'intangible t)
M-: (put-text-property 25 32 'intangible t)

Now there are several places where C-n and C-p get stuck.

Examples for C-n: the `line' of line2; the `l' of line5.

Example for C-p: the `i' of line6; Just after the `3' of line3.

Even if there were no such obvious bugs, there still would be the
question about what the appropriate bug free behavior should be.

What principle should get priority:

1.  Try to avoid skipping lines if there are `tangible' characters on them.

2.  Preserve the column.

For instance, with point on the `1' of line1, C-n goes to the very end
of line3, logical according to principle 2.  According to principle 1,
it should go to the `e' of line2.  The answer really depends on what
the `intangible' property is supposed to be used for.

Sincerely,

Luc.