Re: set-window-margins: lag moving cursor?

[Eli Zaretskii <eliz@gnu.org>]

> From: Marcelo Toledo <marcelo@marcelotoledo.com>
> Date: Wed, 6 Feb 2019 18:09:15 -0200
> 
> When I use really large monitors with GNU Emacs and only one window, I like to add margins left and right to align 100 columns of total width in the middle. Code gets centralized and it’s easier to code.
> 
> Code I am using currently: https://github.com/marcelotoledo/centralized-mode
> Code I was using before: https://github.com/rnkn/olivetti
> 
> What environment I’ve tested:
> 
> * Mac OS X 10.14
> * GNU Emacs 26.1 (latest stable version)
> * Build options: ./configure --build=x86_64-apple-darwin --without-dbus --with-ns
> 
> In that environment I’ve tested speed of cursor moving around random files. 
> 
> How exactly I’ve tested? 
> 
> I’ve set the cursor in the top of the buffer and measured the speed to reach the buttom simply pressing the key binded to <down>. I’ve done that several times and calculated the average (it was pretty steady).
> 
> When I don’t have set-window-margins set it’s moving in a regular speed.
> 
> But whenever have set-window-margins set, it’s easy to notice a ~ 25% reduction of speed moving cursor around.
> 
> I’ve been reading the source code and trying to figure out what exactly is the problem and I was wondering if anyone could help me understand why this is happening and what part of code is bringing this behavior.
> 
> Happy to provide more data.

Thanks.  Please show a complete recipe to reproduce this problem,
starting from "emacs -Q", including how you measured the speed.  A 25%
slowdown is not easy to spot, so I want to be sure I'm doing the same
as you did to see the problem, and measure the same quantity.  Up
front, I don't see why cursor motion would be significantly slower
with margins, but maybe I'm missing something.