bug#29002: it.first_visible_x is erroneously 0 while horizontal scrolling.

[Keith David Bershatsky]

Thank you, Eli, for looking at #29002.

In the screen-shot, line 4 is the current-line and it is hscrolled.

• hscrolling_current_line_p is true.

• w->suspend_auto_hscroll is false.

• w->hscroll > 0.

• All non-current lines are also hscrolled.

• it.first_visible_x == 0.

What would be a good method to programmatically test to see whether all other 
lines are hscrolled given the above?

As I visit each screen line from top to bottom of the window, I will need to 
determine whether the line is hscrolled and then try make my way over to a 
screen relative X coordinate.  move_it_in_display_line_to will need an absolute 
X.

For example, let's say we are on line 6 in the screen-shot.  How do we know 
that it is hscrolled?  If we know for sure it is hscrolled, then we can tell 
move_it_in_display_line_to try and reach (w->hscroll * FRAME_COLUMN_WIDTH (f)) 
+ my_arbitrary_screen_relative_x.

Keith

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DATE:  [10-25-2017 21:51:55] <26 Oct 2017 07:51:55 +0300>
FROM:  Eli Zaretskii <eliz@gnu.org>
> 
>  * * *
> 
> I'm not sure why you expected non-zero values in this case, but if you
> want to identify this situation, use the function
> hscrolling_current_line_p.  Or just calculate the value yourself
> using w->hscroll.