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

Fixes #5979

On Android devices, handwriting input on empty lines triggers childList mutations
when creating new DOM structure for the first character. These mutations get
restored by RestoreDOM during React re-render, which interrupts the IME composition
session and causes premature stroke commits (e.g., writing '我' produces '一我').

The fix adds a targeted check in flush() to skip flushing only when:
1. Currently in composition (IS_COMPOSING is true)
2. Cursor is at the first character of an empty line

This preserves normal onChange behavior for other scenarios while preventing
the handwriting issue on empty lines.

Author: zenggenfa

.changeset/brave-dragons-write.md

@@ -0,0 +1,10 @@
+---
+'slate-react': patch
+---
+
+Fix Android handwriting input where first stroke commits prematurely on empty lines (e.g., '我' becomes '一我').
+
+**Cause:** When writing the first character of an empty line, Android handwriting triggers childList mutations (creating new DOM structure). These mutations get restored by RestoreDOM during React re-render, which interrupts the IME composition session.
+
+**Fix:** Skip flush during composition only when at the first character of an empty line. This targeted fix preserves normal onChange behavior for other scenarios while preventing the handwriting issue.
+

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

@@ -123,7 +123,35 @@ export function createAndroidInputManager({
     action.run()
   }
 
+  // Helper: Check if cursor is at the first character of an empty line
+  const isAtEmptyLineFirstChar = (): boolean => {
+    const { selection, children } = editor
+    if (!selection) return false
+    
+    const { anchor } = selection
+    // offset should be 0 or 1 (first char)
+    if (anchor.offset > 1) return false
+    
+    // Get the block node at current path
+    const blockPath = anchor.path.slice(0, 1) // Get top-level path
+    const blockNode = children[blockPath[0]]
+    if (!blockNode) return true
+    
+    // Check if this line/block is empty or has only one character
+    const text = Node.string(blockNode)
+    return text.length <= 1
+  }
+
   const flush = () => {
+    // FIX: Only skip flush during composition for empty line first char scenario.
+    // Android handwriting input triggers childList mutations when writing the first
+    // character of an empty line. These mutations get restored by RestoreDOM, which
+    // interrupts the IME composition and causes premature commits.
+    // See: https://github.com/ianstormtaylor/slate/issues/5979
+    if (IS_COMPOSING.get(editor) && isAtEmptyLineFirstChar()) {
+      return
+    }
+
     if (flushTimeoutId) {
       clearTimeout(flushTimeoutId)
       flushTimeoutId = null