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

[martin rudalics <rudalics@gmx.at>]

 > How would it detect that, except using the same method it uses now?
 > Note that we are not talking physical lines in the buffer here, we are
 > talking screen lines.  Actually, not even that: we are talking
 > _canonical_ screen lines, i.e. practically pixels.  Now, those 15000
 > lines could well be covered by an invisible property, or by a display
 > property that displays something other than buffer text.  Or they
 > could use a face that calls for a very small font, so that all the
 > 15000 lines take very little screen estate.  In all of these cases, a
 > user who sets scroll-conservatively to 101 wants the screen scrolled
 > rather than recentered.  So that's what try_scrolling tries to do.
 > It's why that function exists.

At least now I understand why setting `scroll-conservatively' to 101 can
be expensive.

 >> But it's redisplay which temporarily puts `point' at EOB and triggers
 >> the fontification subsystem to "work" at that position?
 >
 > No, redisplay doesn't change point, not ever.  It's cc-engine's
 > routines that do it in this case, and I've shown where in the code
 > that happens, see the backtrace I posted.

I see.  Fontification routines just get the boundaries they have to
operate on.  The backtrace is difficult to interpret - I never know
which of Alan's macros save excursion.  But since it returns a buffer
position it likely that it reuses that position later on.

 >> Are you sure that try_scrolling doesn't call this loop over and over
 >> again?
 >
 > Sorry, I don't understand the question.  try_scrolling doesn't include
 > any loops where all this happens, it just calls move_it_to, a single
 > call.  All the rest happens inside that call.  Normally, that call
 > should just descend a few screen lines starting at point (which is at
 > BOB), until it hits the limit I mentioned above, and then
 > try_scrolling should return a failure indication.
 >
 > I did verify that there's a single call to move_it_to, and that all
 > the time you wait is spent inside that call.

I just wanted to make sure that there's no loop in try_scrolling.

 > Why shouldn't CC Mode be aware that point is at BOB?  It has just to
 > look at its value.

It shouldn't matter anyway since, as you said above, the display engine
never sets point.

Thanks for the clarifications.  My questions were naive because I never
looked into how scrolling works and moreover do not understand c-mode
macros well.

martin