tui: Cache the keyboard enhancement check

Wether the host terminal supports keyboard enhancement can be cached
for the lifetime of a Helix session.

Caching this lookup prevents a potential lockup within crossterm's
event reading system where the query for the keyboard enhancement
support waits on the next keyboard event, which can happen if the
crossterm event stream is checked by `tokio::select!` in another
thread.

3 files changed, 14 insertions, 2 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 23127395..a03f9c92 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1202,6 +1202,7 @@ dependencies = [
  "helix-core",
  "helix-view",
  "log",
+ "once_cell",
  "serde",
  "termini",
  "unicode-segmentation",
diff --git a/helix-tui/Cargo.toml b/helix-tui/Cargo.toml
index 3ca7e044..8a6d5367 100644
--- a/helix-tui/Cargo.toml
+++ b/helix-tui/Cargo.toml
@@ -22,6 +22,7 @@ unicode-segmentation = "1.10"
 crossterm = { version = "0.26", optional = true }
 termini = "0.1"
 serde = { version = "1", "optional" = true, features = ["derive"]}
+once_cell = "1.17"
 log = "~0.4"
 helix-view = { version = "0.6", path = "../helix-view", features = ["term"] }
 helix-core = { version = "0.6", path = "../helix-core" }
diff --git a/helix-tui/src/backend/crossterm.rs b/helix-tui/src/backend/crossterm.rs
index e81c1e00..7c512ce3 100644
--- a/helix-tui/src/backend/crossterm.rs
+++ b/helix-tui/src/backend/crossterm.rs
@@ -15,6 +15,7 @@ use crossterm::{
     Command,
 };
 use helix_view::graphics::{Color, CursorKind, Modifier, Rect, UnderlineStyle};
+use once_cell::sync::OnceCell;
 use std::{
     fmt,
     io::{self, Write},
@@ -57,6 +58,7 @@ impl Capabilities {
 pub struct CrosstermBackend<W: Write> {
     buffer: W,
     capabilities: Capabilities,
+    supports_keyboard_enhancement_protocol: OnceCell<bool>,
 }
 
 impl<W> CrosstermBackend<W>
@@ -67,8 +69,16 @@ where
         CrosstermBackend {
             buffer,
             capabilities: Capabilities::from_env_or_default(),
+            supports_keyboard_enhancement_protocol: OnceCell::new(),
         }
     }
+
+    #[inline]
+    fn supports_keyboard_enhancement_protocol(&self) -> io::Result<bool> {
+        self.supports_keyboard_enhancement_protocol
+            .get_or_try_init(terminal::supports_keyboard_enhancement)
+            .copied()
+    }
 }
 
 impl<W> Write for CrosstermBackend<W>
@@ -100,7 +110,7 @@ where
         if config.enable_mouse_capture {
             execute!(self.buffer, EnableMouseCapture)?;
         }
-        if matches!(terminal::supports_keyboard_enhancement(), Ok(true)) {
+        if self.supports_keyboard_enhancement_protocol()? {
             log::debug!("The enhanced keyboard protocol is supported on this terminal");
             execute!(
                 self.buffer,
@@ -121,7 +131,7 @@ where
         if config.enable_mouse_capture {
             execute!(self.buffer, DisableMouseCapture)?;
         }
-        if matches!(terminal::supports_keyboard_enhancement(), Ok(true)) {
+        if self.supports_keyboard_enhancement_protocol()? {
             execute!(self.buffer, PopKeyboardEnhancementFlags)?;
         }
         execute!(