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fix(ui): delay win_viewport until screen update #24182

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Problem:
Sometimes, when nvim sends the `win_viewport` event, for example when scrolling
with visible folds on the screen, it reports the `scroll_delta` value one batch
into "future". So when the client application is trying to show the new viewport
it's not yet updated, resulting in temporary corruption / screen flickering.

For more details see #23609, and starting from [this comment](
https://github.com/neovide/neovide/pull/1790#issuecomment-1518697747) in
https://github.com/neovide/neovide/pull/1790,, where the issue was first
detected. Note that some of the conclusions in those are not fully accurate, but
the general observations are.

Solution:
When there are pending updates to a Window, delay the `win_viewport` UI event
until the updates are sent. This ensures that there's no flush between sending
the viewport and updating of the lines corresponding to the new viewport.

Document the existing viewport behaviour (for cases where there are no
extra flushes), give a hint about how applications can deal with the slightly
surprising behaviour of the viewport event being sent after the updates.

Fixes https://github.com/neovim/neovim/issues/23609
This commit is contained in:
fredizzimo
2023-07-05 16:31:34 +03:00
committed by GitHub
parent 8a788e2daa
commit 1de82e16c1
3 changed files with 10 additions and 2 deletions
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4
src/nvim/window.c
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@@ -1029,7 +1029,9 @@ void ui_ext_win_position(win_T *wp, bool validate)
void ui_ext_win_viewport(win_T *wp)
{
if ((wp == curwin || ui_has(kUIMultigrid)) && wp->w_viewport_invalid) {
// NOTE: The win_viewport command is delayed until the next flush when there are pending updates.
// This ensures that the updates and the viewport are sent together.
if ((wp == curwin || ui_has(kUIMultigrid)) && wp->w_viewport_invalid && wp->w_redr_type == 0) {
const linenr_T line_count = wp->w_buffer->b_ml.ml_line_count;
// Avoid ml_get errors when producing "scroll_delta".
const linenr_T cur_topline = MIN(wp->w_topline, line_count);