ref neovim#7649
ref neovim#7664

27f9b1c 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 27f9b1c 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}~
    ...

Try another approach to defer the termcodes. Seems less janky, but still
not perfect.

ref neovim#7664
ref neovim#7649
ref neovim#7664
ref 27f9b1c

With this implementation there is no "jank" during startup.

Using the main_loop in any fashion is janky. Using only the TUI loop
emits the termcodes too soon, or requires bad hacks like counting
tui_flush invocations (9 seems to work).

ref neovim#7664
ref neovim#7649
ref neovim#7664
ref 27f9b1c

Try another approach to defer the termcodes. Seems less janky, but still
not perfect.

ref neovim#7664
ref neovim#7649
ref neovim#7664
ref 27f9b1c

With this implementation there is no "jank" during startup.

Using the main_loop in any fashion is janky. Using only the TUI loop
emits the termcodes too soon, or requires bad hacks like counting
tui_flush invocations (9 seems to work).

ref neovim#7664
ref neovim#7649
ref neovim#7664
ref 27f9b1c

Try another approach to defer the termcodes. Seems less janky, but still
not perfect.

ref neovim#7664
ref neovim#7649
ref neovim#7664
ref 27f9b1c

With this implementation there is no "jank" during startup.

Using the main_loop in any fashion is janky. Using only the TUI loop
emits the termcodes too soon, or requires bad hacks like counting
tui_flush invocations (9 seems to work).

ref neovim#7664
ref neovim#7649
ref neovim#7664
ref 27f9b1c

- Revert timer-based approach.
- Instead, call loop_poll_events() with a timeout in an "active" loop,
  to infer that "TUI startup activity has mostly finished", but also to
  enforce a mininum time (100 ms) before emitting "enable focus
  reporting" termcode. (If TUI startup takes longer than that minimum
  time, it's probably a slow environment anyways.)
- Tickle `main_loop` by sending a dummy event.  Without this, the
  initial "focus-gained" response from the terminal may not get
  processed until the user hits a key.

ref neovim#7220
ref neovim#7664
ref neovim#7649
ref neovim#7664
ref 27f9b1c

- Revert timer-based approach.
- Instead, call loop_poll_events() with a timeout in an "active" loop,
  to infer that "TUI startup activity has mostly finished", but also to
  enforce a mininum time (100 ms) before emitting "enable focus
  reporting" termcode. (If TUI startup takes longer than that minimum
  time, it's probably a slow environment anyways.)
- Tickle `main_loop` by sending a dummy event.  Without this, the
  initial "focus-gained" response from the terminal may not get
  processed until the user hits a key.

ref neovim#7720
ref neovim#7664
ref neovim#7649
ref neovim#7664
ref 27f9b1c

- Revert timer-based approach.
- Instead, call loop_poll_events() with a timeout in an "active" loop,
  to infer that "TUI startup activity has mostly finished", but also to
  enforce a mininum time (100 ms) before emitting "enable focus
  reporting" termcode. (If TUI startup takes longer than that minimum
  time, it's probably a slow environment anyways.)
- Tickle `main_loop` by sending a dummy event.  Without this, the
  initial "focus-gained" response from the terminal may not get
  processed until the user hits a key.

ref neovim#7720
ref neovim#7664
ref neovim#7649
ref neovim#7664
ref 27f9b1c