Fix IME input when composing in the middle of the textarea

Track the composition range using the textarea selection and avoid re-sending trailing text that existed before composition started (notably in screenReaderMode).

Fixes #5456

src/browser/input/CompositionHelper.ts

@@ -30,6 +30,12 @@ export class CompositionHelper {
    */
   private _compositionPosition: IPosition;
 
+  /**
+   * Text that existed after the composing range when composition started.
+   * This is used to avoid treating existing trailing text as new input.
+   */
+  private _compositionSuffix: string;
+
   /**
    * Whether a composition is in the process of being sent, setting this to false will cancel any
    * in-progress composition.
@@ -57,6 +63,7 @@ export class CompositionHelper {
     this._isComposing = false;
     this._isSendingComposition = false;
     this._compositionPosition = { start: 0, end: 0 };
+    this._compositionSuffix = '';
     this._dataAlreadySent = '';
   }
 
@@ -65,7 +72,11 @@ export class CompositionHelper {
    */
   public compositionstart(): void {
     this._isComposing = true;
-    this._compositionPosition.start = this._textarea.value.length;
+    const start = this._textarea.selectionStart ?? this._textarea.value.length;
+    const end = this._textarea.selectionEnd ?? start;
+    this._compositionPosition.start = Math.min(start, end);
+    this._compositionPosition.end = Math.max(start, end);
+    this._compositionSuffix = this._textarea.value.substring(this._compositionPosition.end);
     this._compositionView.textContent = '';
     this._dataAlreadySent = '';
     this._compositionView.classList.add('active');
@@ -79,7 +90,8 @@ export class CompositionHelper {
     this._compositionView.textContent = ev.data;
     this.updateCompositionElements();
     setTimeout(() => {
-      this._compositionPosition.end = this._textarea.value.length;
+      const end = this._textarea.selectionEnd ?? this._textarea.value.length;
+      this._compositionPosition.end = Math.max( this._compositionPosition.start, end);
     }, 0);
   }
 
@@ -146,6 +158,7 @@ export class CompositionHelper {
         start: this._compositionPosition.start,
         end: this._compositionPosition.end
       };
+      const currentCompositionSuffix = this._compositionSuffix;
 
       // Since composition* events happen before the changes take place in the textarea on most
       // browsers, use a setTimeout with 0ms time to allow the native compositionend event to
@@ -169,10 +182,14 @@ export class CompositionHelper {
             // if a new composition has started.
             input = this._textarea.value.substring(currentCompositionPosition.start, this._compositionPosition.start);
           } else {
-            // Don't use the end position here in order to pick up any characters after the
-            // composition has finished, for example when typing a non-composition character
-            // (eg. 2) after a composition character.
-            input = this._textarea.value.substring(currentCompositionPosition.start);
+            // Keep support for non-composition characters typed immediately after composition end
+            // while avoiding re-sending the trailing text that was already present
+            // before composition started.
+            const value = this._textarea.value;
+            const valueEnd = currentCompositionSuffix.length > 0 && value.endsWith(currentCompositionSuffix)
+              ? value.length - currentCompositionSuffix.length
+              : value.length;
+            input = value.substring(currentCompositionPosition.start, Math.max(currentCompositionPosition.start, valueEnd));
           }
           if (input.length > 0) {
             this._coreService.triggerDataEvent(input, true);