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>

1 files changed, 20 insertions, 1 deletions

diff --git a/src/protocol/wire.rs b/src/protocol/wire.rs
index 8225cb8c..e31f8ff5 100644
--- a/src/protocol/wire.rs
+++ b/src/protocol/wire.rs
@@ -13,7 +13,7 @@ use serde::{Deserialize, Serialize};
 // ---------------------------------------------------------------------------
 
 /// Current protocol version. Bumped when wire format changes incompatibly.
-pub const PROTOCOL_VERSION: u32 = 15;
+pub const PROTOCOL_VERSION: u32 = 16;
 
 /// Maximum allowed frame payload size (2 MB). Frames larger than this are
 /// rejected to prevent denial-of-service via oversized length prefixes.
@@ -656,6 +656,14 @@ pub enum ServerMessage {
         /// True when Herdr mouse UI is enabled or the focused pane app requests mouse reporting.
         enabled: bool,
     },
+
+    /// Apply the prefix-mode ASCII input-source change on the foreground client.
+    /// `active = true` → switch to an ASCII-capable source (saving the current one);
+    /// `active = false` → restore the saved source.
+    PrefixInputSource {
+        /// Whether the ASCII input source should be active.
+        active: bool,
+    },
 }
 
 // ---------------------------------------------------------------------------
@@ -1404,6 +1412,17 @@ mod tests {
         assert_eq!(msg, decoded);
     }
 
+    #[test]
+    fn server_prefix_input_source_roundtrip() {
+        for active in [true, false] {
+            let msg = ServerMessage::PrefixInputSource { active };
+            let encoded = bincode::serde::encode_to_vec(&msg, bincode::config::standard()).unwrap();
+            let (decoded, _): (ServerMessage, _) =
+                bincode::serde::decode_from_slice(&encoded, bincode::config::standard()).unwrap();
+            assert_eq!(msg, decoded);
+        }
+    }
+
     // ---- Framing ----
 
     #[test]