tui.c: request focus-reporting (fix regression) #7670

ref #7649
ref #7664

27f9b1c7b0 caused a regression: it uses loop_schedule_deferred() to
defer emitting the "enable focus reporting" termcode. tui_main() never
processes `tui_loop.events` (which loop_schedule_deferred() depends on),
so the event was never actually processed.

But fixing that (by processing `tui_loop.events`) would bring back the
problem 27f9b1c7b0 tried to fix: it still emits the event too soon.

Instead, do a little dance: schedule the event on `main_loop` and then
forward it to `tui_loop`.

NOTE: after this commit, in tmux 2.3 with `focus-events` enabled,
FocusGained is fired on startup and when resuming from suspend.

Using `script` to record the terminal session (and `vterm-dump` to
post-process the result):

BEFORE:
    {DECSM 1049}{DECSM 1}{ESC =}
    {CUP *}{ED *}{DECSM 2004}{DECSM 1004}{CSI 1,43 r}
    {CUP 1,1}
    {CUP *}{ED *}{SM 34}{DECSM 25}
    {DECRM 25}{CSI 2   q}{CSI 2   q}
    {CUP *}{ED *}{LF}
    {SGR *}{LS1}{SGR 94}~
    ...

AFTER:
    {CUP *}{ED *}{CSI 1,43 r}
    {CUP 1,1}
    {CUP *}{ED *}{SM 34}{DECSM 25}
    {DECRM 25}{CSI 2   q}{CSI 2   q}
    {CUP *}{ED *}{DECSM 2004}{DECSM 1004}{LF}
    {SGR *}{LS1}{SGR 94}~
    ...

src/nvim/event/loop.h

@@ -19,12 +19,11 @@ typedef struct loop {
   MultiQueue *events;
   MultiQueue *thread_events;
   // Immediate events:
-  //    "Events that should be processed after exiting uv_run() (to avoid
-  //    recursion), but before returning from loop_poll_events()."
-  //    502aee690c980fcb3cfcb3f211dcfad06103db46
-  // Practical consequence: these events are processed by
+  //    "Processed after exiting uv_run() (to avoid recursion), but before
+  //    returning from loop_poll_events()." 502aee690c98
+  // Practical consequence (for main_loop): these events are processed by
   //    state_enter()..os_inchar()
-  // whereas "regular" (main_loop.events) events are processed by
+  // whereas "regular" events (main_loop.events) are processed by
   //    state_enter()..VimState.execute()
   // But state_enter()..os_inchar() can be "too early" if you want the event
   // to trigger UI updates and other user-activity-related side-effects.

src/nvim/tui/tui.c

@@ -169,10 +169,16 @@ static size_t unibi_pre_fmt_str(TUIData *data, unsigned int unibi_index,
 
 /// Emits some termcodes after Nvim startup, which were observed to slowdown
 /// rendering during startup in tmux 2.3 (+focus-events). #7649
-static void terminfo_start_event(void **argv)
+static void terminfo_after_startup_event(void **argv)
 {
   UI *ui = argv[0];
+  bool defer = argv[1] != NULL;  // clever(?) boolean without malloc() dance.
   TUIData *data = ui->data;
+  if (defer) {  // We're on the main-loop. Now forward to the TUI loop.
+    loop_schedule(data->loop,
+                  event_create(terminfo_after_startup_event, 2, ui, NULL));
+    return;
+  }
   // Enable bracketed paste
   unibi_out_ext(ui, data->unibi_ext.enable_bracketed_paste);
   // Enable focus reporting
@@ -268,6 +274,9 @@ static void terminfo_start(UI *ui)
     uv_pipe_init(&data->write_loop, &data->output_handle.pipe, 0);
     uv_pipe_open(&data->output_handle.pipe, data->out_fd);
   }
+
+  loop_schedule(&main_loop,
+                event_create(terminfo_after_startup_event, 2, ui, ui));
 }
 
 static void terminfo_stop(UI *ui)
@@ -304,8 +313,6 @@ static void tui_terminal_start(UI *ui)
   update_size(ui);
   signal_watcher_start(&data->winch_handle, sigwinch_cb, SIGWINCH);
   term_input_start(&data->input);
-  loop_schedule_deferred(data->loop,
-                         event_create(terminfo_start_event, 1, ui));
 }
 
 static void tui_terminal_stop(UI *ui)
@@ -352,7 +359,7 @@ static void tui_main(UIBridgeData *bridge, UI *ui)
   CONTINUE(bridge);
 
   while (!data->stop) {
-    loop_poll_events(&tui_loop, -1);
+    loop_poll_events(&tui_loop, -1);  // tui_loop.events is never processed
   }
 
   ui_bridge_stopped(bridge);