Re: Problems with move_it_in_display_line_to X when tabs exist.

[Keith David Bershatsky]

I could use a little help, please, to perform the same test that you did.  I 
built Emacs 26 tonight _without_ any modifications for both OSX and Windows.  I 
launched Emacs 26 on both platforms with no user configuration using gdb.  From 
gdb, I set a breakpoint as follows:

break xdisp.c:22746

I created a long word-wrapped line with a single space at the right window 
edge, followed by a continued line with three (3) xxx, followed by a tab, 
followed by hello-world.  I evaluated the following code (using setq instead of 
set for the tab-width):

(setq buffer-display-table (make-display-table))
(aset buffer-display-table
      ?\t
      (vector (make-glyph-code ?\u00BB 'font-lock-warning-face)
              (make-glyph-code ?\t 'fringe)))
(setq word-wrap t)
(setq tab-width 2)
(setq visual-order-cursor-movement t)

I placed by my cursor on the 00BB character (aka »), and I typed M-x right-char.

On both platforms, IT is on the second "x" from the far left of the window 
edge.  In other words, I appear to be moving in the wrong direction and when I 
go to print the it.*** info in gdb, I am clearly on the second "x" -- i.e., I 
am one character to the _left_ of ».

Thanks,

Keith

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DATE:  [12-03-2017 06:29:57] <03 Dec 2017 16:29:57 +0200>
FROM:  Eli Zaretskii <address@hidden>
> 
> * * *
> 
> The above snippet leaves a lot undefined, so I simulated that as
> follows:
> 
>   (setq buffer-display-table (make-display-table))
>   (aset buffer-display-table
>         ?\t
>         (vector (make-glyph-code ?\u00BB 'font-lock-warning-face)
>                 (make-glyph-code ?\t 'fringe)))
>   (setq word-wrap t)
>   (set tab-width 2)
> 
> Then I inserted a long line as you described, arranging for its second
> screen line to start with "xxx" followed by a TAB.
> 
> Then I emulated your calls to move_it_in_display_line_to by setting
> visual-order-cursor-movement to t, putting the cursor on the 0xBB
> character which depicts the TAB, and typing "M-x right-char RET".
> If I set a breakpoint in xdisp.c:22746, which is here:
> 
>       else if (it.current_x != target_x)
>  move_it_in_display_line_to (&it, ZV, target_x, MOVE_TO_POS | MOVE_TO_X);
> 
> I see that this call returns with it.current on the stretch glyph
> following the 0xBB character, and with it.pixel_width set correctly to
> the width of the stretch glyph.
> 
> So I think either you call move_it_in_display_line_to in some way that
> is different from the above, or there's something else you didn't tell
> in your recipe.