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Fix: Prevent flicker when LSP server is slow #18439

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girishji
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Ref: girishji/vimcomplete#101 (comment)

In insert-mode completion, the leader text is temporarily deleted while searching for completion candidates.
If the LSP server responds slowly, the client may call :sleep to wait, which triggers out_flush().
This causes the deleted text to briefly disappear before being redrawn when results arrive, producing a visible flicker.

There are two possible fixes:

  1. Suppress flushing while a user function (e.g. LSP client) is waiting.
  2. Reinsert the deleted text before invoking the user function.

This PR implements (1), which is the simpler solution, though somewhat heavy-handed. If you think this may introduce unwanted side effects, I can rework it to use (2).

@girishji
Fix: Prevent flicker when LSP server is slow
f63a12d 
In insert-mode completion, the leader text is temporarily deleted while
searching for completion candidates.
If the LSP server responds slowly, the client may call `:sleep` to wait,
which triggers `out_flush()`.
This causes the deleted text to briefly disappear before being redrawn when
results arrive, producing a visible flicker.

There are two possible fixes:

1. Suppress flushing while a user function (e.g. LSP client) is waiting.
2. Reinsert the deleted text before invoking the user function.

This PR implements (1), which is the simpler solution, though somewhat
heavy-handed. If you think this may introduce unwanted side effects, I can
rework it to use (2).
@girishji
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278574c 
M  src/insexpand.c
@girishji
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a520363 
M  src/insexpand.c
@chrisbra
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thanks

@chrisbra chrisbra closed this in 71b97f2 Sep 29, 2025
@habamax
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habamax commented Sep 29, 2025

Thx. It is better and worse at the same time :).

@habamax
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habamax commented Sep 29, 2025

Screencast doesn't show, but there are noticeable delays between keypress and char insertion, which feels not great.

@girishji
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I'll look into this.

@girishji girishji deleted the blink branch September 30, 2025 01:35
@chrisbra
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That patch is wrong. It causes overflow of out_pos, because when no_flush is set, out_char_nf() happily writes out-of-bound because the out_pos counter is no longer reset when out_flush() is called. I'll revert this for now.

It causes segfault on MS-Windows: #18450

Stack trace:

==377936==T0: stack [0x000000e40000,0x000000f40000) size 0x100000; local=0x000000f3eec0
==377936==AddressSanitizer Init done
=================================================================
==377936==ERROR: AddressSanitizer: global-buffer-overflow on address 0x7ff61ef5c0c0 at pc 0x7ff61eb98668 bp 0x000000f35560 sp 0x000000f35568
WRITE of size 1 at 0x7ff61ef5c0c0 thread T0
    #0 0x7ff61eb98667 in out_str_nf c:\Users\Christian Brabandt\code\vim\src\term.c:2893
    #1 0x7ff61eb9dd06 in out_str c:\Users\Christian Brabandt\code\vim\src\term.c:2987
    #2 0x7ff61eb134fc in windgoto c:\Users\Christian Brabandt\code\vim\src\screen.c:3213
    #3 0x7ff61eb18907 in screen_char c:\Users\Christian Brabandt\code\vim\src\screen.c:1978
    #4 0x7ff61eb1513d in screen_puts_len c:\Users\Christian Brabandt\code\vim\src\screen.c:1501
    #5 0x7ff61ea0be19 in screen_puts_mbyte c:\Users\Christian Brabandt\code\vim\src\message.c:2238
    #6 0x7ff61ea0b2fb in msg_puts_display c:\Users\Christian Brabandt\code\vim\src\message.c:2553
    #7 0x7ff61ea0bcd8 in msg_puts_attr_len c:\Users\Christian Brabandt\code\vim\src\message.c:2362
    #8 0x7ff61ea0feb1 in msg_outtrans_len_attr c:\Users\Christian Brabandt\code\vim\src\message.c:1816
    #9 0x7ff61ea0dd4a in msg_outtrans_attr c:\Users\Christian Brabandt\code\vim\src\message.c:1696
    #10 0x7ff61ea0dd4a in msg_attr_keep c:\Users\Christian Brabandt\code\vim\src\message.c:192
    #11 0x7ff61ea10448 in msg_attr c:\Users\Christian Brabandt\code\vim\src\message.c:139
    #12 0x7ff61ea10448 in msg_source c:\Users\Christian Brabandt\code\vim\src\message.c:553
    #13 0x7ff61ea0d45e in emsg_core c:\Users\Christian Brabandt\code\vim\src\message.c:782
    #14 0x7ff61ebca4e6 in undo_allowed c:\Users\Christian Brabandt\code\vim\src\undo.c:338
    #15 0x7ff61e9b4a83 in f_complete c:\Users\Christian Brabandt\code\vim\src\insexpand.c:3877
    #16 0x7ff61e8ec29c in call_internal_func c:\Users\Christian Brabandt\code\vim\src\evalfunc.c:3497
    #17 0x7ff61ebe8cdb in call_func c:\Users\Christian Brabandt\code\vim\src\userfunc.c:4176
    #18 0x7ff61ebe9d75 in get_func_tv c:\Users\Christian Brabandt\code\vim\src\userfunc.c:2190
    #19 0x7ff61ebd4c73 in ex_call_inner c:\Users\Christian Brabandt\code\vim\src\userfunc.c:6510
    #20 0x7ff61ebe5264 in ex_call c:\Users\Christian Brabandt\code\vim\src\userfunc.c:6868
    #21 0x7ff61e92a52c in do_one_cmd c:\Users\Christian Brabandt\code\vim\src\ex_docmd.c:2629
    #22 0x7ff61e931b04 in do_cmdline c:\Users\Christian Brabandt\code\vim\src\ex_docmd.c:1041
    #23 0x7ff61ebdb3d5 in call_user_func c:\Users\Christian Brabandt\code\vim\src\userfunc.c:3312
    #24 0x7ff61ebda2b1 in call_user_func_check c:\Users\Christian Brabandt\code\vim\src\userfunc.c:3485
    #25 0x7ff61ebe909c in call_func c:\Users\Christian Brabandt\code\vim\src\userfunc.c:4158
    #26 0x7ff61ebe9d75 in get_func_tv c:\Users\Christian Brabandt\code\vim\src\userfunc.c:2190
    #27 0x7ff61ebd4c73 in ex_call_inner c:\Users\Christian Brabandt\code\vim\src\userfunc.c:6510
    #28 0x7ff61ebe5264 in ex_call c:\Users\Christian Brabandt\code\vim\src\userfunc.c:6868
    #29 0x7ff61e92a52c in do_one_cmd c:\Users\Christian Brabandt\code\vim\src\ex_docmd.c:2629
    #30 0x7ff61e931b04 in do_cmdline c:\Users\Christian Brabandt\code\vim\src\ex_docmd.c:1041
    #31 0x7ff61ebdb3d5 in call_user_func c:\Users\Christian Brabandt\code\vim\src\userfunc.c:3312
    #32 0x7ff61ebda2b1 in call_user_func_check c:\Users\Christian Brabandt\code\vim\src\userfunc.c:3485
    #33 0x7ff61ebe909c in call_func c:\Users\Christian Brabandt\code\vim\src\userfunc.c:4158
    #34 0x7ff61ebeb943 in call_callback c:\Users\Christian Brabandt\code\vim\src\userfunc.c:3818
    #35 0x7ff61e9ac80a in expand_by_function c:\Users\Christian Brabandt\code\vim\src\insexpand.c:3612
    #36 0x7ff61e9a7b68 in get_next_completion_match c:\Users\Christian Brabandt\code\vim\src\insexpand.c:5357
    #37 0x7ff61e9a6d6e in ins_compl_get_exp c:\Users\Christian Brabandt\code\vim\src\insexpand.c:5599
    #38 0x7ff61e9a59b2 in find_next_completion_match c:\Users\Christian Brabandt\code\vim\src\insexpand.c:6151
    #39 0x7ff61e9a55a9 in ins_compl_next c:\Users\Christian Brabandt\code\vim\src\insexpand.c:6265
    #40 0x7ff61e9b6eee in ins_complete c:\Users\Christian Brabandt\code\vim\src\insexpand.c:7320
    #41 0x7ff61e8b6ff4 in edit c:\Users\Christian Brabandt\code\vim\src\edit.c:1329
    #42 0x7ff61ea3139a in invoke_edit c:\Users\Christian Brabandt\code\vim\src\normal.c:7139
    #43 0x7ff61ea318e9 in nv_edit c:\Users\Christian Brabandt\code\vim\src\normal.c:7110
    #44 0x7ff61ea43efc in normal_cmd c:\Users\Christian Brabandt\code\vim\src\normal.c:952
    #45 0x7ff61e92d809 in exec_normal c:\Users\Christian Brabandt\code\vim\src\ex_docmd.c:9436
    #46 0x7ff61e8e70b3 in f_feedkeys c:\Users\Christian Brabandt\code\vim\src\evalfunc.c:5151
    #47 0x7ff61e8ec29c in call_internal_func c:\Users\Christian Brabandt\code\vim\src\evalfunc.c:3497
    #48 0x7ff61ebe8cdb in call_func c:\Users\Christian Brabandt\code\vim\src\userfunc.c:4176
    #49 0x7ff61ebe9d75 in get_func_tv c:\Users\Christian Brabandt\code\vim\src\userfunc.c:2190
    #50 0x7ff61ebd4c73 in ex_call_inner c:\Users\Christian Brabandt\code\vim\src\userfunc.c:6510
    #51 0x7ff61ebe5264 in ex_call c:\Users\Christian Brabandt\code\vim\src\userfunc.c:6868
    #52 0x7ff61e92a52c in do_one_cmd c:\Users\Christian Brabandt\code\vim\src\ex_docmd.c:2629
    #53 0x7ff61e931b04 in do_cmdline c:\Users\Christian Brabandt\code\vim\src\ex_docmd.c:1041
    #54 0x7ff61e930e83 in do_cmdline_cmd c:\Users\Christian Brabandt\code\vim\src\ex_docmd.c:635
    #55 0x7ff61eba1977 in f_assert_fails c:\Users\Christian Brabandt\code\vim\src\testing.c:624
    #56 0x7ff61e8ec29c in call_internal_func c:\Users\Christian Brabandt\code\vim\src\evalfunc.c:3497
    #57 0x7ff61ebe8cdb in call_func c:\Users\Christian Brabandt\code\vim\src\userfunc.c:4176
    #58 0x7ff61ebe9d75 in get_func_tv c:\Users\Christian Brabandt\code\vim\src\userfunc.c:2190
    #59 0x7ff61ebd4c73 in ex_call_inner c:\Users\Christian Brabandt\code\vim\src\userfunc.c:6510
    #60 0x7ff61ebe5264 in ex_call c:\Users\Christian Brabandt\code\vim\src\userfunc.c:6868
    #61 0x7ff61e92a52c in do_one_cmd c:\Users\Christian Brabandt\code\vim\src\ex_docmd.c:2629
    #62 0x7ff61e931b04 in do_cmdline c:\Users\Christian Brabandt\code\vim\src\ex_docmd.c:1041
    #63 0x7ff61ebdb3d5 in call_user_func c:\Users\Christian Brabandt\code\vim\src\userfunc.c:3312
    #64 0x7ff61ebda2b1 in call_user_func_check c:\Users\Christian Brabandt\code\vim\src\userfunc.c:3485
    #65 0x7ff61ebe909c in call_func c:\Users\Christian Brabandt\code\vim\src\userfunc.c:4158
    #66 0x7ff61ebe9d75 in get_func_tv c:\Users\Christian Brabandt\code\vim\src\userfunc.c:2190
    #67 0x7ff61ebd4c73 in ex_call_inner c:\Users\Christian Brabandt\code\vim\src\userfunc.c:6510
    #68 0x7ff61ebe5264 in ex_call c:\Users\Christian Brabandt\code\vim\src\userfunc.c:6868
    #69 0x7ff61e92a52c in do_one_cmd c:\Users\Christian Brabandt\code\vim\src\ex_docmd.c:2629
    #70 0x7ff61e931b04 in do_cmdline c:\Users\Christian Brabandt\code\vim\src\ex_docmd.c:1041
    #71 0x7ff61ea3ddb3 in nv_colon c:\Users\Christian Brabandt\code\vim\src\normal.c:3170
    #72 0x7ff61ea43efc in normal_cmd c:\Users\Christian Brabandt\code\vim\src\normal.c:952
    #73 0x7ff61e9d07e9 in main_loop c:\Users\Christian Brabandt\code\vim\src\main.c:1635
    #74 0x7ff61e9cf31e in vim_main2 c:\Users\Christian Brabandt\code\vim\src\main.c:973
    #75 0x7ff61e9cfd8b in VimMain c:\Users\Christian Brabandt\code\vim\src\main.c:446
    #76 0x7ff61ec7322f in wWinMain c:\Users\Christian Brabandt\code\vim\src\os_w32exe.c:39
    #77 0x7ff61e7e35b1 in invoke_main D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:118
    #78 0x7ff61e7e35b1 in __scrt_common_main_seh D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288
    #79 0x7ffc5f3de8d6  (C:\Windows\System32\KERNEL32.DLL+0x18002e8d6)
    #80 0x7ffc60d68d9b  (C:\Windows\SYSTEM32\ntdll.dll+0x180008d9b)

0x7ff61ef5c0c0 is located 0 bytes after global variable 'out_buf' defined in 'term.c:2759:15' (0x7ff61ef5b8c0) of size 2048
SUMMARY: AddressSanitizer: global-buffer-overflow c:\Users\Christian Brabandt\code\vim\src\term.c:2893 in out_str_nf
Shadow bytes around the buggy address:
  0x7ff61ef5be00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7ff61ef5be80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7ff61ef5bf00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7ff61ef5bf80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x7ff61ef5c000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x7ff61ef5c080: 00 00 00 00 00 00 00 00[f9]f9 f9 f9 f9 f9 f9 f9
  0x7ff61ef5c100: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
  0x7ff61ef5c180: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
  0x7ff61ef5c200: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
  0x7ff61ef5c280: f9 f9 f9 f9 f9 f9 f9 f9 04 f9 f9 f9 04 f9 f9 f9
  0x7ff61ef5c300: 00 f9 f9 f9 04 f9 f9 f9 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==377936==ABORTING

@girishji
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girishji commented Oct 1, 2025 •
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There are side effects indeed. Should be reverted.

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3 participants
@girishji @chrisbra @habamax