bug#43587: move-to-column behaves differently when text has invisible property

[Christoph Arenz <tiga.arenz@web.de>]

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Digging deeper into bug 43587, it seems to be related to move-to-column
behaving slightly different when the text in a line has the invisible
property set.

Here is how to recreate. According to the documentation for
move-to-column, I would not expect any difference between the two lines:

 > Optional second argument FORCE non-nil means if COLUMN is in the
middle of a tab character, change it to spaces.

For the first line, the tab has not been changed to spaces...

Am I overlooking something?

Kind Regards,
Christoph

(progn
   (switch-to-buffer "indent-test.txt")
   (erase-buffer)
   (insert "\tLine starting with INVISIBLE text after TAB\n")
   (insert "\tLine starting with visible text after TAB\n")
   (insert "\nUsing move-to-column to move 'into' TAB, using the FORCE
parameter on both lines\n")
   (whitespace-mode 1)
   (add-text-properties 2 21 '(invisible t))
   (beginning-of-buffer)
   (move-to-column 7 t)
   (forward-line)
   (move-to-column 7 t))


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