bug#42406: Mouse-wheel scrolling can be flickering

[Alan Third]

On Sun, Dec 13, 2020 at 09:05:29PM +0200, Eli Zaretskii wrote:
> > Date: Fri, 11 Dec 2020 20:37:56 +0000
> > From: Alan Third <alan@idiocy.org>
> > Cc: konrad.podczeck@univie.ac.at, 42406@debbugs.gnu.org
> > 
> > > Any pointers to the code which causes all the frames to be updated in
> > > C-v/M-v case?
> > 
> > I don't know. I'm looking at redisplay_internal in a debugger and I
> > can see that consider_all_windows_p is true, which will be because
> > windows_or_buffers_changed == 2 == REDISPLAY_SOME or because
> > update_mode_lines == 42.
> > 
> > I can't find anywhere that sets update_mode_lines to 42...
> 
> wset_update_mode_line does it, so any caller of that function would.
> 
> But where do you see the value of 42 being tested?

I've got two frames open, I set up a breakpoint in
ns_clear_under_internal_border and scroll. When I hit the breakpoint
for the frame I don't expect to be updated I get this backtrace:

* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
  * frame #0: 0x00000001001eb35f 
temacs`ns_clear_under_internal_border(f=0x000000010711cea0) at nsterm.m:3036:7 
[opt]
    frame #1: 0x0000000100029cd5 temacs`redisplay_internal at xdisp.c:16030:21 
[opt]
    frame #2: 0x00000001000cb1d5 temacs`read_char(commandflag=1, 
map=0x0000000107d635c3, prev_event=0x0000000000000000, 
used_mouse_menu=0x00007ffeefbff36b, end_time=0x0000000000000000) at 
keyboard.c:2496:6 [opt]
    frame #3: 0x00000001000c8e72 temacs`read_key_sequence(keybuf=<unavailable>, 
prompt=0x0000000000000000, dont_downcase_last=<unavailable>, 
can_return_switch_frame=<unavailable>, fix_current_buffer=<unavailable>, 
prevent_redisplay=<unavailable>) at keyboard.c:9483:12 [opt]
    frame #4: 0x00000001000c756c temacs`command_loop_1 at keyboard.c:1353:15 
[opt]
    frame #5: 0x000000010014dd17 
temacs`internal_condition_case(bfun=(temacs`command_loop_1 at keyboard.c:1239), 
handlers=0x0000000000000090, hfun=(temacs`cmd_error at keyboard.c:922)) at 
eval.c:1415:25 [opt]
    frame #6: 0x00000001000d7670 temacs`command_loop_2(ignore=<unavailable>) at 
keyboard.c:1094:11 [opt]
    frame #7: 0x000000010014d53b temacs`internal_catch(tag=0x000000000000c9f0, 
func=(temacs`command_loop_2 at keyboard.c:1090), arg=0x0000000000000000) at 
eval.c:1176:25 [opt]
    frame #8: 0x0000000100212935 temacs`command_loop.cold.1 + 69
    frame #9: 0x00000001000c6633 temacs`command_loop at keyboard.c:1072:2 [opt]
    frame #10: 0x00000001000c6563 temacs`recursive_edit_1 at keyboard.c:720:9 
[opt]
    frame #11: 0x00000001000c67bb temacs`Frecursive_edit at keyboard.c:789:3 
[opt]
    frame #12: 0x00000001000c5343 temacs`main(argc=<unavailable>, 
argv=0x00007ffeefbff810) at emacs.c:2054:3 [opt]
    frame #13: 0x00007fff712cd3d5 libdyld.dylib`start + 1* thread #1, queue = 
'com.apple.main-thread', stop reason = breakpoint 1.1
  * frame #0: 0x00000001001eb35f 
temacs`ns_clear_under_internal_border(f=0x000000010711cea0) at nsterm.m:3036:7 
[opt]
    frame #1: 0x0000000100029cd5 temacs`redisplay_internal at xdisp.c:16030:21 
[opt]
    frame #2: 0x00000001000cb1d5 temacs`read_char(commandflag=1, 
map=0x0000000107d635c3, prev_event=0x0000000000000000, 
used_mouse_menu=0x00007ffeefbff36b, end_time=0x0000000000000000) at 
keyboard.c:2496:6 [opt]
    frame #3: 0x00000001000c8e72 temacs`read_key_sequence(keybuf=<unavailable>, 
prompt=0x0000000000000000, dont_downcase_last=<unavailable>, 
can_return_switch_frame=<unavailable>, fix_current_buffer=<unavailable>, 
prevent_redisplay=<unavailable>) at keyboard.c:9483:12 [opt]
    frame #4: 0x00000001000c756c temacs`command_loop_1 at keyboard.c:1353:15 
[opt]
    frame #5: 0x000000010014dd17 
temacs`internal_condition_case(bfun=(temacs`command_loop_1 at keyboard.c:1239), 
handlers=0x0000000000000090, hfun=(temacs`cmd_error at keyboard.c:922)) at 
eval.c:1415:25 [opt]
    frame #6: 0x00000001000d7670 temacs`command_loop_2(ignore=<unavailable>) at 
keyboard.c:1094:11 [opt]
    frame #7: 0x000000010014d53b temacs`internal_catch(tag=0x000000000000c9f0, 
func=(temacs`command_loop_2 at keyboard.c:1090), arg=0x0000000000000000) at 
eval.c:1176:25 [opt]
    frame #8: 0x0000000100212935 temacs`command_loop.cold.1 + 69
    frame #9: 0x00000001000c6633 temacs`command_loop at keyboard.c:1072:2 [opt]
    frame #10: 0x00000001000c6563 temacs`recursive_edit_1 at keyboard.c:720:9 
[opt]
    frame #11: 0x00000001000c67bb temacs`Frecursive_edit at keyboard.c:789:3 
[opt]
    frame #12: 0x00000001000c5343 temacs`main(argc=<unavailable>, 
argv=0x00007ffeefbff810) at emacs.c:2054:3 [opt]
    frame #13: 0x00007fff712cd3d5 libdyld.dylib`start + 1

Which I've not found very useful. I had a look at redisplay_internal
to see if I could work out why the frame is being updated, but I
can't.

I looked at some of the variables in redisplay_internal that are
involved in making the decision whether to check all frames:

(lldb) p update_mode_lines;
(int) $0 = 42
(lldb) p windows_or_buffers_changed
(int) $1 = 2

but I have no idea if this is useful at all.

Is there anything specific you'd like me to check?
-- 
Alan Third