Re: "... the window start at a meaningless point within a line."

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 30 Sep 2015 20:45:13 +0000
> From: Alan Mackenzie <acm@muc.de>
> 
> In the Elisp manual, page "Window Start and End", it's described how, if
> in (set-window-start x) x is a position not at the start of a line, the
> display engine will, instead of x, choose some value near x as the
> window start.  Is this really necessary?

The reason for this is that we want the physical beginning of the line
(i.e. the character after a newline or at bob) always be aligned at
the left edge of the window's text area, even if it is currently out
of view.  This prevents annoying horizontal scrolling of long lines
when the beginning of the line comes into view.

> The particular problem I'm looking at is with Follow Mode, where a pair
> of adjacent windows are of unequal width.  This is a very common
> scenario - I frequently use Follow Mode with three side by side windows
> of widths 79, 79, and 80 (as measured by window-body-width).
> 
> The problem occurs when there's a long line at the bottom of window 2
> and its continuation line at the top of window 3.  The display manager
> chops off the line in W2 after 78 characters, and inserts a \ sign.  It
> then starts the continuation line in W3 at character 80 (counting from
> 1), assuming as it does, that the previous line in this window would be
> 79 characters long.  So character 79 appears neither on the first
> portion of the line in W2, nor in its continuation in W3.  This is
> suboptimal.
> 
> This problem would not occur if there were a flag to set-window-start
> meaning "this value really is where I want the window to start, not some
> beginning of line nearby".
> 
> How difficult would it be to relax the constraint on window start
> positions and implement this flag?

It's not difficult, at least in principle: the recalculation of the
window-start point is done in a single function.  Not sure if it will
have adverse effects, though.  You will surely have that horizontal
scrolling, but I don't know what other effects will be caused.

But let me turn the table and ask you: how difficult would it be to
make sure follow-mode never lets a window end in the middle of a
continued line?