fix(slate-react): prevent premature flush during composition for Android handwriting input

Fixes #5979

On Android devices, handwriting input triggers compositionStart/compositionEnd
events per stroke. The flush() function was being called during composition,
causing DOM changes that interrupt the composition session. This resulted in
the first stroke being committed prematurely (e.g., writing '我' would produce '一我').

The fix adds IS_COMPOSING checks in flush(), scheduleAction(), and handleInput()
to prevent flushing during composition. The pending diffs are now properly
accumulated and flushed only after compositionEnd.

Author: zenggenfa

.changeset/brave-dragons-write.md

@@ -0,0 +1,10 @@
+---
+'slate-react': patch
+---
+
+Fix Android handwriting input where first stroke commits prematurely (e.g., '我' becomes '一我').
+
+**Cause:** Android handwriting triggers compositionStart/compositionEnd per stroke. The flush() was being called during composition, causing DOM changes that interrupted the session.
+
+**Fix:** Added IS_COMPOSING check at the start of flush() to skip flushing during composition. Pending changes are now properly accumulated and flushed after compositionEnd.
+

packages/slate-react/src/hooks/android-input-manager/android-input-manager.ts

@@ -124,6 +124,13 @@ export function createAndroidInputManager({
   }
 
   const flush = () => {
+    // FIX: Don't flush during composition to prevent DOM changes that interrupt
+    // the composition session. This fixes Android handwriting input issue.
+    // See: https://github.com/ianstormtaylor/slate/issues/5979
+    if (IS_COMPOSING.get(editor)) {
+      return
+    }
+
     if (flushTimeoutId) {
       clearTimeout(flushTimeoutId)
       flushTimeoutId = null