On Wayland, fix invalid offsets being sent in Preedit

Even when the protocol explicitly tells to send proper UTF-8
boundaries for cursor, some IMEs don't do that, so sanity check
them before sending downstream.

CHANGELOG.md

@@ -8,6 +8,8 @@ And please only add new entries to the top of this list, right below the `# Unre
 
 # Unreleased
 
+- On Wayland, fix byte offset in `Ime::Preedit` pointing to invalid bytes.
+
 # 0.27.4
 
 - On Windows, emit `ReceivedCharacter` events on system keybindings.

src/platform_impl/linux/wayland/seat/text_input/handlers.rs

@@ -68,9 +68,14 @@ pub(super) fn handle_text_input(
             cursor_begin,
             cursor_end,
         } => {
-            let cursor_begin = usize::try_from(cursor_begin).ok();
-            let cursor_end = usize::try_from(cursor_end).ok();
             let text = text.unwrap_or_default();
+            let cursor_begin = usize::try_from(cursor_begin)
+                .ok()
+                .and_then(|idx| text.is_char_boundary(idx).then(|| idx));
+            let cursor_end = usize::try_from(cursor_end)
+                .ok()
+                .and_then(|idx| text.is_char_boundary(idx).then(|| idx));
+
             inner.pending_preedit = Some(Preedit {
                 text,
                 cursor_begin,