fix: switch prefix ASCII input source on the foreground client (#1016)

* fix: switch prefix ASCII input source on the foreground client

The macOS prefix ASCII-switch ran in the headless server, whose
TISCopyCurrentKeyboardInputSource read is a stale per-process cache
(refreshed only via a main-run-loop notification the server never pumps),
so prefix could leave the terminal on the wrong input source.

Forward it to the foreground client, like clipboard: sync_prefix_input_source
emits AppEvent::PrefixInputSource as an intent; the server forwards it to the
foreground client and the monolithic app applies it in-process via
RealPrefixInputSource, which pumps its main run loop before the stale TIS read.
Key it on Mode::wants_ascii_input so multi-level prefix commands keep ASCII
until returning to the terminal, and restore the IME for rename text entry.

Adds ServerMessage::PrefixInputSource to the already-unreleased protocol 15.

refs #774

* fix: keep prefix input-source switch out of the headless server process

An App-internal drain (the exhaustive drain at the top of
handle_api_request, reachable from prefix-key runtime mutations) can
consume a queued PrefixInputSource intent before the server's
forwarding drain sees it, applying the TIS switch in the headless
server process and stranding the restore state. Gate the in-process
switch on App.local_input_source_switch, disabled by the headless
server alongside the other local side effects, so a swallowed intent
degrades to a skipped switch. Widen the forwarding-bypass maintenance
test to the handle_internal_event_with_prefix_sync wrapper.

refs #774

* docs: clarify prefix input-source comments and flag docs

refs #774

* fix: bump protocol for prefix input source

refs #774

---------

Co-authored-by: Ogulcan Celik <ogulcancelik@gmail.com>

2 files changed, 34 insertions, 0 deletions

diff --git a/src/platform/macos.rs b/src/platform/macos.rs
index 3770a3be..e72d32fe 100644
--- a/src/platform/macos.rs
+++ b/src/platform/macos.rs
@@ -97,6 +97,34 @@ extern "C" {
         buffer_size: CfIndex,
         encoding: u32,
     ) -> Boolean;
+
+    #[link_name = "kCFRunLoopDefaultMode"]
+    static CF_RUN_LOOP_DEFAULT_MODE: CfStringRef;
+
+    #[link_name = "CFRunLoopRunInMode"]
+    fn cf_run_loop_run_in_mode(
+        mode: CfStringRef,
+        seconds: f64,
+        return_after_source_handled: Boolean,
+    ) -> libc::c_int;
+}
+
+/// Pump the main thread's run loop once (non-blocking) so the process receives the
+/// `kTISNotifySelectedKeyboardInputSourceChanged` notification and refreshes the per-process cache
+/// that `TISCopyCurrentKeyboardInputSource` reads. That notification arrives only via the main
+/// thread's run loop, so a process that never runs a CFRunLoop (the headless server) reads a stale
+/// source. Must run on the main thread.
+pub(crate) fn pump_input_source_runloop() {
+    debug_assert!(
+        // SAFETY: `pthread_main_np` is always safe to call.
+        unsafe { libc::pthread_main_np() } != 0,
+        "pump_input_source_runloop must run on the main thread"
+    );
+    // SAFETY: `CFRunLoopRunInMode` is thread-safe; a 0-second call drains the ready sources and
+    // returns immediately (no blocking). `CF_RUN_LOOP_DEFAULT_MODE` is a framework-owned constant.
+    unsafe {
+        let _ = cf_run_loop_run_in_mode(CF_RUN_LOOP_DEFAULT_MODE, 0.0, 0);
+    }
 }
 
 #[derive(Debug)]
diff --git a/src/platform/mod.rs b/src/platform/mod.rs
index 169b7c1c..889c4ce9 100644
--- a/src/platform/mod.rs
+++ b/src/platform/mod.rs
@@ -154,6 +154,9 @@ pub(crate) fn switch_to_ascii_input_source() -> Option<InputSourceRestore> {
     None
 }
 
+#[cfg(not(target_os = "macos"))]
+pub(crate) fn pump_input_source_runloop() {}
+
 /// Switches the host keyboard input source while prefix mode is active.
 ///
 /// `App` drives this through a trait so the prefix-mode transitions can be
@@ -179,6 +182,9 @@ pub(crate) struct RealPrefixInputSource {
 impl PrefixInputSource for RealPrefixInputSource {
     fn switch_to_ascii(&mut self) {
         if self.restore.is_none() {
+            // Drain pending input-source-change notifications so the read below is fresh (see
+            // `pump_input_source_runloop`); a no-op on non-macOS.
+            pump_input_source_runloop();
             self.restore = switch_to_ascii_input_source();
         }
     }