bug#30584: Tab stretches vanish when horizontal scrolling.

[Eli Zaretskii]

> Date: Thu, 22 Feb 2018 14:42:56 -0800
> From: Keith David Bershatsky <esq@lawlist.com>
> 
> Step 1.  Launch Emacs 26 from the terminal so that we can observe STDERR 
> messages when dumping the glyph row.
> 
> Step 2.  Switch to the *scratch* buffer.
> 
> Step 3.  Evaluate the following Lisp code:
> 
>     (setq display-line-numbers t)
>     (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 'highlight)))
> 
> Step 4.  On a new line, type:  C-q TAB C-q TAB Hello-world.
> 
> Step 5.  Evaluate:  (scroll-left 2)
> 
> Step 6.  M-x dump-glyph-row
> 
> We observe that the _second_ tab STRETCH, which was visible immediately 
> _before_ Step 5, is now missing.  We see that the tab STRETCH that is missing 
> has a dump glyph row width of -1.  If we were to look at the value of 
> it.pixel_width, it would be equal to negative the value of a regular width 
> character; i.e., -7.

Thanks, should be fixed now on the master branch.