Re: Scrolling commands and skipping redisplay, was: Re: emacs rendering comparisson between emacs23 and emacs26.3

[Stefan Monnier]

> When the last repetition is executed, the behavior seems fine already if the
> value is 0. What I see is "flickering" during scrolling, while C-v is still
> pressed.

Hmmm... I don't know why you see "flickering" during the scrolling, indeed.

And I'm surprised you don't see "flickering" on the last repetition: the
way jit-lock-defer-time works, when input-pending-p is non-nil, jit-lock
marks the chunk as fontified (which is a lie), and this chunk will
always go past `window-end`, so on the last repetition the beginning of
the window is not passed to jit-lock at all (because it was part of the
last chunk of the previous call to jit-lock, with input-pending = true),
and it only gets fontified a bit later via the timer.

That's one of the main advantages of the way
`fast-but-imprecise-scrolling` works: it doesn't lie about having
fontified that chunk.

Along the lines of what you proposed, I just tried the patch below,
which is a bit like (setq jit-lock-defer-mode 0) but without the idle timer.
It's not quite right either, tho (e.g. `emacs -Q src/xdisp.c` fails to font-lock
the initial window, tho it does get font-locked as soon as I move the
cursor).


        Stefan


diff --git a/src/xdisp.c b/src/xdisp.c
index 193cc372b0..230e8e2182 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -4097,6 +4097,7 @@ handle_fontified_prop (struct it *it)
 {
   Lisp_Object prop, pos;
   enum prop_handled handled = HANDLED_NORMALLY;
+  extern bool input_was_pending;
 
   if (!NILP (Vmemory_full))
     return handled;
@@ -4108,6 +4109,7 @@ handle_fontified_prop (struct it *it)
   if (!STRINGP (it->string)
       && it->s == NULL
       && !NILP (Vfontification_functions)
+      && !input_was_pending
       && !NILP (Vrun_hooks)
       && (pos = make_fixnum (IT_CHARPOS (*it)),
          prop = Fget_char_property (pos, Qfontified, Qnil),