fix(tui): do not toggle cursor visibility when flushing the buffer (#26055)

When writing large amounts of data to the tty it is common to first hide
the cursor to avoid a flickering effect. This has been done in Nvim for
a long time and was implemented in the function that actually flushed
the TUI buffer out to the TTY.

However, when using synchronized updates with the 'termsync' option this
is no longer necessary, as the terminal emulator will buffer all of the
updates and display them atomically. Thus there is no need to toggle the
cursor visibility when flushing the buffer when synchronized updates are
used. In fact, doing so can actually reintroduce cursor flickering in
certain scenarios because the visibility state is itself being
synchronized by the terminal.

In addition, the management of the cursor visibility should not happen
when the TUI _buffer_ is flushed, but rather when the TUI itself is
flushed. This is a subtle but meaningful distinction: the former
literally writes bytes to the TTY while the latter flushes the TUI's
grid into its buffer. There is no need to hide the cursor every time we
write bytes to the TTY, only at the beginning of a full TUI "flush"
event.

src/nvim/tui/tui.c

@@ -75,9 +75,6 @@ struct TUIData {
   unibi_var_t params[9];
   char buf[OUTBUF_SIZE];
   size_t bufpos;
-  char norm[CNORM_COMMAND_MAX_SIZE];
-  char invis[CNORM_COMMAND_MAX_SIZE];
-  size_t normlen, invislen;
   TermInput input;
   uv_loop_t write_loop;
   unibi_term *ut;
@@ -210,19 +207,11 @@ void tui_enable_extkeys(TUIData *tui)
   unibi_out_ext(tui, tui->unibi_ext.enable_extended_keys);
 }
 
-static size_t unibi_pre_fmt_str(TUIData *tui, unsigned unibi_index, char *buf, size_t len)
-{
-  const char *str = unibi_get_str(tui->ut, unibi_index);
-  if (!str) {
-    return 0U;
-  }
-  return unibi_run(str, tui->params, buf, len);
-}
-
 /// Request the terminal's DEC mode (DECRQM).
 ///
 /// @see handle_modereport
 static void tui_dec_request_mode(TUIData *tui, TerminalDecMode mode)
+  FUNC_ATTR_NONNULL_ALL
 {
   // 5 bytes for \x1b[?$p, 1 byte for null terminator, 6 bytes for mode digits (more than enough)
   char buf[12];
@@ -233,8 +222,8 @@ static void tui_dec_request_mode(TUIData *tui, TerminalDecMode mode)
 
 /// Handle a DECRPM response from the terminal.
 void tui_dec_report_mode(TUIData *tui, TerminalDecMode mode, TerminalModeState state)
+  FUNC_ATTR_NONNULL_ALL
 {
-  assert(tui);
   switch (state) {
   case kTerminalModeNotRecognized:
   case kTerminalModePermanentlySet:
@@ -352,10 +341,6 @@ static void terminfo_start(TUIData *tui)
     || terminfo_is_term_family(term, "win32con")
     || terminfo_is_term_family(term, "interix");
   tui->bce = unibi_get_bool(tui->ut, unibi_back_color_erase);
-  tui->normlen = unibi_pre_fmt_str(tui, unibi_cursor_normal,
-                                   tui->norm, sizeof tui->norm);
-  tui->invislen = unibi_pre_fmt_str(tui, unibi_cursor_invisible,
-                                    tui->invis, sizeof tui->invis);
   // Set 't_Co' from the result of unibilium & fix_terminfo.
   t_colors = unibi_get_num(tui->ut, unibi_max_colors);
   // Enter alternate screen, save title, and clear.
@@ -1151,9 +1136,7 @@ void tui_set_mode(TUIData *tui, ModeShape mode)
     HlAttrs aep = kv_A(tui->attrs, c.id);
 
     tui->want_invisible = aep.hl_blend == 100;
-    if (tui->want_invisible) {
+    if (!tui->want_invisible && aep.rgb_ae_attr & HL_INVERSE) {
-      unibi_out(tui, unibi_cursor_invisible);
-    } else if (aep.rgb_ae_attr & HL_INVERSE) {
       // We interpret "inverse" as "default" (no termcode for "inverse"...).
       // Hopefully the user's default cursor color is inverse.
       unibi_out_ext(tui, tui->unibi_ext.reset_cursor_color);
@@ -1174,6 +1157,14 @@ void tui_set_mode(TUIData *tui, ModeShape mode)
     unibi_out_ext(tui, tui->unibi_ext.reset_cursor_color);
   }
 
+  if (tui->want_invisible && !tui->is_invisible) {
+    unibi_out(tui, unibi_cursor_invisible);
+    tui->is_invisible = true;
+  } else if (!tui->want_invisible && tui->is_invisible) {
+    unibi_out(tui, unibi_cursor_normal);
+    tui->is_invisible = false;
+  }
+
   int shape;
   switch (c.shape) {
   case SHAPE_BLOCK:
@@ -1305,18 +1296,30 @@ void tui_default_colors_set(TUIData *tui, Integer rgb_fg, Integer rgb_bg, Intege
   invalidate(tui, 0, tui->grid.height, 0, tui->grid.width);
 }
 
-/// Enable synchronized output. When enabled, the terminal emulator will preserve the last rendered
+/// Begin flushing the TUI. If 'termsync' is set and the terminal supports synchronized updates,
-/// state on subsequent re-renders. It will continue to process incoming events. When synchronized
+/// begin a synchronized update. Otherwise, hide the cursor to avoid cursor jumping.
-/// mode is disabled again the emulator renders using the most recent state. This avoids tearing
+static void tui_flush_start(TUIData *tui)
-/// when the terminal updates the screen faster than Nvim can redraw it.
+  FUNC_ATTR_NONNULL_ALL
-static void tui_sync_output(TUIData *tui, bool enable)
 {
-  if (!tui->sync_output) {
+  if (tui->sync_output && tui->unibi_ext.sync != -1) {
-    return;
+    UNIBI_SET_NUM_VAR(tui->params[0], 1);
+    unibi_out_ext(tui, tui->unibi_ext.sync);
+  } else {
+    unibi_out(tui, unibi_cursor_invisible);
   }
+}
 
-  UNIBI_SET_NUM_VAR(tui->params[0], enable ? 1 : 0);
+/// Finish flushing the TUI. If 'termsync' is set and the terminal supports synchronized updates,
-  unibi_out_ext(tui, tui->unibi_ext.sync);
+/// end a synchronized update. Otherwise, make the cursor visible again.
+static void tui_flush_end(TUIData *tui)
+  FUNC_ATTR_NONNULL_ALL
+{
+  if (tui->sync_output && tui->unibi_ext.sync != -1) {
+    UNIBI_SET_NUM_VAR(tui->params[0], 0);
+    unibi_out_ext(tui, tui->unibi_ext.sync);
+  } else if (!tui->busy && !tui->want_invisible) {
+    unibi_out(tui, unibi_cursor_normal);
+  }
 }
 
 void tui_flush(TUIData *tui)
@@ -1335,7 +1338,7 @@ void tui_flush(TUIData *tui)
     tui_busy_stop(tui);  // avoid hidden cursor
   }
 
-  tui_sync_output(tui, true);
+  tui_flush_start(tui);
 
   while (kv_size(tui->invalid_regions)) {
     Rect r = kv_pop(tui->invalid_regions);
@@ -1364,7 +1367,7 @@ void tui_flush(TUIData *tui)
 
   cursor_goto(tui, tui->row, tui->col);
 
-  tui_sync_output(tui, false);
+  tui_flush_end(tui);
 
   flush_buf(tui);
 }
@@ -2252,63 +2255,20 @@ static void augment_terminfo(TUIData *tui, const char *term, int vte_version, in
 static void flush_buf(TUIData *tui)
 {
   uv_write_t req;
-  uv_buf_t bufs[3];
+  uv_buf_t buf;
-  uv_buf_t *bufp = &bufs[0];
 
-  // The content of the output for each condition is shown in the following
+  if (tui->bufpos <= 0) {
-  // table. Therefore, if tui->bufpos == 0 and N/A or invis + norm, there is
-  // no need to output it.
-  //
-  //                         | is_invisible | !is_invisible
-  // ------+-----------------+--------------+---------------
-  // busy  | want_invisible  |     N/A      |    invis
-  //       | !want_invisible |     N/A      |    invis
-  // ------+-----------------+--------------+---------------
-  // !busy | want_invisible  |     N/A      |    invis
-  //       | !want_invisible |     norm     | invis + norm
-  // ------+-----------------+--------------+---------------
-  //
-  if (tui->bufpos <= 0
-      && ((tui->is_invisible && tui->busy)
-          || (tui->is_invisible && !tui->busy && tui->want_invisible)
-          || (!tui->is_invisible && !tui->busy && !tui->want_invisible))) {
     return;
   }
 
-  if (!tui->is_invisible) {
+  buf.base = tui->buf;
-    // cursor is visible. Write a "cursor invisible" command before writing the
+  buf.len = UV_BUF_LEN(tui->bufpos);
-    // buffer.
-    bufp->base = tui->invis;
-    bufp->len = UV_BUF_LEN(tui->invislen);
-    bufp++;
-    tui->is_invisible = true;
-  }
-
-  if (tui->bufpos > 0) {
-    bufp->base = tui->buf;
-    bufp->len = UV_BUF_LEN(tui->bufpos);
-    bufp++;
-  }
-
-  if (!tui->busy) {
-    assert(tui->is_invisible);
-    // not busy and the cursor is invisible. Write a "cursor normal" command
-    // after writing the buffer.
-    if (!tui->want_invisible) {
-      bufp->base = tui->norm;
-      bufp->len = UV_BUF_LEN(tui->normlen);
-      bufp++;
-      tui->is_invisible = false;
-    }
-  }
 
   if (tui->screenshot) {
-    for (size_t i = 0; i < (size_t)(bufp - bufs); i++) {
+    fwrite(buf.base, buf.len, 1, tui->screenshot);
-      fwrite(bufs[i].base, bufs[i].len, 1, tui->screenshot);
-    }
   } else {
     int ret
-      = uv_write(&req, (uv_stream_t *)&tui->output_handle, bufs, (unsigned)(bufp - bufs), NULL);
+      = uv_write(&req, (uv_stream_t *)&tui->output_handle, &buf, 1, NULL);
     if (ret) {
       ELOG("uv_write failed: %s", uv_strerror(ret));
     }