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.

Cargo.lock

@@ -1202,6 +1202,7 @@ dependencies = [
  "helix-core",
  "helix-view",
  "log",
+ "once_cell",
  "serde",
  "termini",
  "unicode-segmentation",

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" }

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!(