fix(tui): avoid flushing buffer halfway an OSC 2 sequence (#30793)

Problem:  Setting title while TUI buffer is almost full may cause the
          end of a flush to be treated as a part of an OSC 2 or OSC 0
          sequence, leading to problems like invisible cursor.
Solution: Make the whole sequence to set title a unibi_ext string.

src/nvim/tui/tui.c

@@ -134,11 +134,12 @@ struct TUIData {
     int resize_screen;
     int reset_scroll_region;
     int set_cursor_style, reset_cursor_style;
-    int save_title, restore_title;
+    int save_title, restore_title, set_title;
     int set_underline_style;
     int set_underline_color;
     int sync;
   } unibi_ext;
+  char *set_title;
   char *space_buf;
   size_t space_buf_len;
   bool stopped;
@@ -536,6 +537,7 @@ static void terminfo_stop(TUIData *tui)
     abort();
   }
   unibi_destroy(tui->ut);
+  XFREE_CLEAR(tui->set_title);
 }
 
 static void tui_terminal_start(TUIData *tui)
@@ -1567,8 +1569,7 @@ void tui_suspend(TUIData *tui)
 
 void tui_set_title(TUIData *tui, String title)
 {
-  if (!(unibi_get_str(tui->ut, unibi_to_status_line)
+  if (!unibi_get_ext_str(tui->ut, (unsigned)tui->unibi_ext.set_title)) {
-        && unibi_get_str(tui->ut, unibi_from_status_line))) {
     return;
   }
   if (title.size > 0) {
@@ -1577,9 +1578,9 @@ void tui_set_title(TUIData *tui, String title)
       unibi_out_ext(tui, tui->unibi_ext.save_title);
       tui->title_enabled = true;
     }
-    unibi_out(tui, unibi_to_status_line);
+    UNIBI_SET_NUM_VAR(tui->params[0], 0);
-    out(tui, title.data, title.size);
+    UNIBI_SET_STR_VAR(tui->params[1], title.data);
-    unibi_out(tui, unibi_from_status_line);
+    unibi_out_ext(tui, tui->unibi_ext.set_title);
   } else if (tui->title_enabled) {
     // Restore title/icon from the "stack". #4063
     unibi_out_ext(tui, tui->unibi_ext.restore_title);
@@ -1803,12 +1804,17 @@ static void unibi_goto(TUIData *tui, int row, int col)
       memset(&vars, 0, sizeof(vars)); \
       tui->cork = true; \
 retry: \
+      /* Copy parameters on every retry, as unibi_format() may modify them. */ \
       memcpy(params, tui->params, sizeof(params)); \
       unibi_format(vars, vars + 26, str, params, out, tui, pad, tui); \
       if (tui->overflow) { \
         tui->bufpos = orig_pos; \
-        flush_buf(tui); \
+        /* If orig_pos is 0, there's nothing to flush and retrying won't work. */ \
-        goto retry; \
+        /* TODO(zeertzjq): should this situation still be handled? */ \
+        if (orig_pos > 0) { \
+          flush_buf(tui); \
+          goto retry; \
+        } \
       } \
       tui->cork = false; \
     } \
@@ -1840,6 +1846,7 @@ static void out(void *ctx, const char *str, size_t len)
     }
     flush_buf(tui);
   }
+  // TODO(zeertzjq): handle string longer than buffer size? #30794
 
   memcpy(tui->buf + tui->bufpos, str, len);
   tui->bufpos += len;
@@ -2378,6 +2385,19 @@ static void augment_terminfo(TUIData *tui, const char *term, int vte_version, in
   tui->unibi_ext.save_title = (int)unibi_add_ext_str(ut, "ext.save_title", "\x1b[22;0t");
   tui->unibi_ext.restore_title = (int)unibi_add_ext_str(ut, "ext.restore_title", "\x1b[23;0t");
 
+  const char *tsl = unibi_get_str(ut, unibi_to_status_line);
+  const char *fsl = unibi_get_str(ut, unibi_from_status_line);
+  if (tsl != NULL && fsl != NULL) {
+    // Add a single extended capability for the whole sequence to set title,
+    // as it is usually an OSC sequence that cannot be cut in half.
+    // Use %p2 for the title string, as to_status_line may take an argument.
+    size_t set_title_len = strlen(tsl) + strlen("%p2%s") + strlen(fsl);
+    char *set_title = xmallocz(set_title_len);
+    snprintf(set_title, set_title_len + 1, "%s%s%s", tsl, "%p2%s", fsl);
+    tui->unibi_ext.set_title = (int)unibi_add_ext_str(ut, "ext.set_title", set_title);
+    tui->set_title = set_title;
+  }
+
   /// Terminals usually ignore unrecognized private modes, and there is no
   /// known ambiguity with these. So we just set them unconditionally.
   tui->unibi_ext.enable_lr_margin =

test/functional/terminal/tui_spec.lua

@@ -1856,6 +1856,7 @@ describe('TUI', function()
     retry(nil, nil, function()
       eq({ true, 330 }, { child_session:request('nvim_win_get_height', 0) })
     end)
+    child_session:request('nvim_set_option_value', 'cursorline', true, {})
     -- Use full screen message so that redrawing afterwards is more deterministic.
     child_session:notify('nvim_command', 'intro')
     screen:expect({ any = 'Nvim is open source and freely distributable' })
@@ -1865,14 +1866,7 @@ describe('TUI', function()
     -- The whole line needs 3 + 9 + 3 * 21838 + 3 = 65529 bytes.
     -- The cursor_address that comes after will overflow the 65535-byte buffer.
     local line = ('Ꝩ'):rep(21838) .. '℃'
-    child_session:notify(
+    child_session:notify('nvim_buf_set_lines', 0, 0, -1, true, { line, 'b' })
-      'nvim_exec_lua',
-      [[
-      vim.api.nvim_buf_set_lines(0, 0, -1, true, {...})
-      vim.o.cursorline = true
-    ]],
-      { line, 'b' }
-    )
     -- Close the :intro message and redraw the lines.
     feed_data('\n')
     screen:expect([[
@@ -1887,6 +1881,46 @@ describe('TUI', function()
     ]])
   end)
 
+  it('draws correctly when setting title overflows #30793', function()
+    screen:try_resize(67, 327)
+    retry(nil, nil, function()
+      eq({ true, 324 }, { child_session:request('nvim_win_get_height', 0) })
+    end)
+    child_exec_lua([[
+      vim.o.cmdheight = 0
+      vim.o.laststatus = 0
+      vim.o.ruler = false
+      vim.o.showcmd = false
+      vim.o.termsync = false
+      vim.o.title = true
+    ]])
+    retry(nil, nil, function()
+      eq('[No Name] - Nvim', api.nvim_buf_get_var(0, 'term_title'))
+      eq({ true, 326 }, { child_session:request('nvim_win_get_height', 0) })
+    end)
+    -- Use full screen message so that redrawing afterwards is more deterministic.
+    child_session:notify('nvim_command', 'intro')
+    screen:expect({ any = 'Nvim is open source and freely distributable' })
+    -- Going to top-left corner needs 3 bytes.
+    -- A Ꝩ character takes 3 bytes.
+    -- The whole line needs 3 + 3 * 21842 = 65529 bytes.
+    -- The title will be updated because the buffer is now modified.
+    -- The start of the OSC 0 sequence to set title can fit in the 65535-byte buffer,
+    -- but the title string cannot.
+    local line = ('Ꝩ'):rep(21842)
+    child_session:notify('nvim_buf_set_lines', 0, 0, -1, true, { line })
+    -- Close the :intro message and redraw the lines.
+    feed_data('\n')
+    screen:expect([[
+      {1:Ꝩ}ꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨ|
+      ꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨꝨ|*325
+      {3:-- TERMINAL --}                                                     |
+    ]])
+    retry(nil, nil, function()
+      eq('[No Name] + - Nvim', api.nvim_buf_get_var(0, 'term_title'))
+    end)
+  end)
+
   it('visual bell (padding) does not crash #21610', function()
     feed_data ':set visualbell\n'
     screen:expect {