The boolean variable truncate-lines controls whether continuation lines are displayed or not. I use this when I am checking some log file where the very long lines are truncated thanks to a previous (setq truncate-lines t). This is convenient since the relevant contents is usually at the beginning of the lines. However I sometimes need to see the end of the inspected lines and I'll occasionally toggle truncate-lines on and off (this is done in code temporarily bound to a function key).

The problem is that toggling truncate-lines on and off does not update the display unless I do some editing, or move the cursor far enough, or .. I don't know what. I tried to insert a (redisplay t) in my toggling code but this is not enough.

What bit of elisp magic will cause the display to be updated according to the new value of truncate-lines once it is changed? And, by the way, is the lack of immediate update a bona fide bug?

How to reproduce the problem: After some investigation, it seems that the problem appears because I invoke the toggle via some key binding and not by execute-extended-command aka M-x. Here is a test file for you to download. Start with emacs -Q test_file, then eval-defun the first expression. Now M-x ttl does the toggle ok but on my machine hitting CTRL l (now bound to ttl) does the toggle without redisplay. If I move to end of buffer, the redisplay is activated. FWIW I use emacs 24.5.1

Added Jan. 5th: it turns out that the behaviour described above is due to a minor bug that will be fixed in Emacs 25, see here. In principle there is nothing wrong with changing truncate-lines directly without going through toggle-truncate-lines.

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    Can you provide a recipe starting from emacs -Q to reproduce the problem? I toggle-truncate-lines a lot, and I don't recall it ever failing to have an immediate effect on the visual state of the buffer. – phils Jan 3 '16 at 12:07
  • I've now just done that. Thanks for the suggestion. It turns out that the redisplay works ok when I toggle via execute-extended-command aka M-x. It fails when I toggle via key bindings. – phs Jan 3 '16 at 13:12
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    You should use toggle-truncate-lines rather than changing directly the value of truncate-lines. (global-set-key "\C-l" 'toggle-truncate-lines) does work. doc – JeanPierre Jan 3 '16 at 13:51
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    @JeanPierre: that's probably the answer. Could you change your comment to an answer? – Dan Jan 3 '16 at 19:58
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    phs: You can M-x report-emacs-bug to suggest a documentation change. – phils Jan 4 '16 at 8:47

You should use the command toggle-truncate-lines rather than changing directly the value of variabletruncate-lines. This command does ensure the display is updated (apparently through force-mode-line-update although nor its name neither its docstring does suggest that).

It is described in emacs manual but unfortunately not mentionned in truncate-lines docstring nor in the section about truncation of elisp manual.

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