On X11, use events modifiers to detect state

While there's a separate event to deliver modifiers for keyboard,
unfortunately, it's not even remotely reflects the modifiers state.

Thus use events along side regular modifier updates to correctly
detect the state. Also, apply the modifiers from the regular
key event by converting their state to xkb modifiers state.

Links: https://github.com/alacritty/alacritty/issues/7549
Closes: #3388

src/platform_impl/linux/common/xkb/compose.rs

@@ -0,0 +1,124 @@
+//! XKB compose handling.
+
+use std::env;
+use std::ffi::CString;
+use std::ops::Deref;
+use std::os::unix::ffi::OsStringExt;
+use std::ptr::NonNull;
+
+use super::XkbContext;
+use super::XKBCH;
+use smol_str::SmolStr;
+use xkbcommon_dl::{
+    xkb_compose_compile_flags, xkb_compose_feed_result, xkb_compose_state, xkb_compose_state_flags,
+    xkb_compose_status, xkb_compose_table, xkb_keysym_t,
+};
+
+#[derive(Debug)]
+pub struct XkbComposeTable {
+    table: NonNull<xkb_compose_table>,
+}
+
+impl XkbComposeTable {
+    pub fn new(context: &XkbContext) -> Option<Self> {
+        let locale = env::var_os("LC_ALL")
+            .and_then(|v| if v.is_empty() { None } else { Some(v) })
+            .or_else(|| env::var_os("LC_CTYPE"))
+            .and_then(|v| if v.is_empty() { None } else { Some(v) })
+            .or_else(|| env::var_os("LANG"))
+            .and_then(|v| if v.is_empty() { None } else { Some(v) })
+            .unwrap_or_else(|| "C".into());
+        let locale = CString::new(locale.into_vec()).unwrap();
+
+        let table = unsafe {
+            (XKBCH.xkb_compose_table_new_from_locale)(
+                context.as_ptr(),
+                locale.as_ptr(),
+                xkb_compose_compile_flags::XKB_COMPOSE_COMPILE_NO_FLAGS,
+            )
+        };
+
+        let table = NonNull::new(table)?;
+        Some(Self { table })
+    }
+
+    /// Create new state with the given compose table.
+    pub fn new_state(&self) -> Option<XkbComposeState> {
+        let state = unsafe {
+            (XKBCH.xkb_compose_state_new)(
+                self.table.as_ptr(),
+                xkb_compose_state_flags::XKB_COMPOSE_STATE_NO_FLAGS,
+            )
+        };
+
+        let state = NonNull::new(state)?;
+        Some(XkbComposeState { state })
+    }
+}
+
+impl Deref for XkbComposeTable {
+    type Target = NonNull<xkb_compose_table>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.table
+    }
+}
+
+impl Drop for XkbComposeTable {
+    fn drop(&mut self) {
+        unsafe {
+            (XKBCH.xkb_compose_table_unref)(self.table.as_ptr());
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct XkbComposeState {
+    state: NonNull<xkb_compose_state>,
+}
+
+impl XkbComposeState {
+    pub fn get_string(&mut self, scratch_buffer: &mut Vec<u8>) -> Option<SmolStr> {
+        super::make_string_with(scratch_buffer, |ptr, len| unsafe {
+            (XKBCH.xkb_compose_state_get_utf8)(self.state.as_ptr(), ptr, len)
+        })
+    }
+
+    #[inline]
+    pub fn feed(&mut self, keysym: xkb_keysym_t) -> ComposeStatus {
+        let feed_result = unsafe { (XKBCH.xkb_compose_state_feed)(self.state.as_ptr(), keysym) };
+        match feed_result {
+            xkb_compose_feed_result::XKB_COMPOSE_FEED_IGNORED => ComposeStatus::Ignored,
+            xkb_compose_feed_result::XKB_COMPOSE_FEED_ACCEPTED => {
+                ComposeStatus::Accepted(self.status())
+            }
+        }
+    }
+
+    #[inline]
+    pub fn reset(&mut self) {
+        unsafe {
+            (XKBCH.xkb_compose_state_reset)(self.state.as_ptr());
+        }
+    }
+
+    #[inline]
+    pub fn status(&mut self) -> xkb_compose_status {
+        unsafe { (XKBCH.xkb_compose_state_get_status)(self.state.as_ptr()) }
+    }
+}
+
+impl Drop for XkbComposeState {
+    fn drop(&mut self) {
+        unsafe {
+            (XKBCH.xkb_compose_state_unref)(self.state.as_ptr());
+        };
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+pub enum ComposeStatus {
+    Accepted(xkb_compose_status),
+    Ignored,
+    None,
+}