Composing text shouldn't be part of undo/redo (#108765)

Fixes undo/redo behavior when using a composing keyboard, especially on desktop.

Author: justinmc

packages/flutter/lib/src/widgets/editable_text.dart

@@ -4712,6 +4712,24 @@ class _TextEditingHistoryState extends State<_TextEditingHistory> {
       return;
     }
 
+    switch (defaultTargetPlatform) {
+      case TargetPlatform.iOS:
+      case TargetPlatform.macOS:
+      case TargetPlatform.fuchsia:
+      case TargetPlatform.linux:
+      case TargetPlatform.windows:
+        // Composing text is not counted in history coalescing.
+        if (!widget.controller.value.composing.isCollapsed) {
+          return;
+        }
+        break;
+      case TargetPlatform.android:
+        // Gboard on Android puts non-CJK words in composing regions. Coalesce
+        // composing text in order to allow the saving of partial words in that
+        // case.
+        break;
+    }
+
     _throttleTimer = _throttledPush(widget.controller.value);
   }