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bug#16526: 24.3.50; scroll-conservatively & c-mode regression

From: martin rudalics
Subject: bug#16526: 24.3.50; scroll-conservatively & c-mode regression
Date: Sat, 25 Jan 2014 19:25:32 +0100

>> Well ... so you know why it calls back_comment around the END of the
>> buffer?
> It starts at the end of the buffer, but then moves all the way back
> to the beginning.

IIUC `beginning-of-buffer' does set_point_both.  Does that move all the
way back to the beginning?

> And yes, I know why: it's because scroll-conservatively causes
> redisplay to examine buffer text around point,

Where is `point' at that time?

> when it decides where
> to place window-start.  This is triggered by redisplay after the move
> to the end of the buffer.

But the `end-of-buffer' call terminates cleanly after a few seconds -
that's what the `sit-for' proves in my code.

>>  > What I see is that find_defun_start is called many times,
>> ... 530 times as I mentioned earlier ...
>>  > with its
>>  > first argument moving from _end_ of the buffer backwards.
>> Not monotonously.  Sometimes it's called from the same position (for
>> example 948653 is at least three times on my list) again.
> True.  But I'm not sure this is relevant to the slow redisplay.

It hints at some really bad code embedded in really bad code.

>>  > This
>>  > happens when Emacs needs to redisplay the last portion of the buffer,
>>  > immediately after the call to end-of-buffer.
>> Hmm ... but the problem is when going to BOB.
> No, going to BOB is instantaneous.  The problem happens in redisplay
> after going to EOB.

EOB happens before my `sit-for'.

>>  > JIT Lock is triggered also when font-lock is turned on after the move
>>  > to end of the buffer.  But the difference seems to come from the fact
>>  > that under scroll-conservatively, we examine the buffer a little bit
>>  > above/below the window, when we decide where to put window-start.
>> And somehow a "current position" is still near the end of the buffer at
>> that time.
> Yes, because Emacs is at EOB.



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