Re: The future of Follow Mode - a proposal.

[Alan Mackenzie <acm@muc.de>]

Hello, Stefan.

On Thu, Feb 25, 2016 at 04:10:22PM -0500, Stefan Monnier wrote:
> > Merely controlling the order of window display is no great feat.
> > Determining window starts for these Follow Mode windows is the difficult
> > bit.

> If you con control the order, you can make sure the first follow-mode
> window is rendered first, so you can set the window-start of the second
> window before it gets rendered, etc...
> IOW, controlling the order of window display lets you solve the issue of
> determining the window-start for subsequent windows.

But often it is the second or subsequent window that is to be fixed in
place (say, after a scroll down operation in that window).  The
difficult bit is to determine the starting position of the first window
which will result in the subsequent window being correctly placed when
there is to be no gap, and no overlap between windows.

Follow Mode currently does a lot of backwards and forwards motion (e.g.
with `vertical-motion') to get this right.  The display engine could do
this much more efficiently - Lisp has access only to a small part of the
functionality in C.

> > Currently Follow Mode fights with the display engine to get what
> > it needs, often resulting in delays of several tenths of a second in the
> > subsequent redisplay operations after the first one.

> That's why I'd hope that if you add a hook so follow-mode can tweak the
> redisplay from inside, it should work much better.

Possibly.  What would this hook do?  Calculating the start position of
an earlier window is probably best done in C (for speed).

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).