diff --git a/src/browser/input/CompositionHelper.test.ts b/src/browser/input/CompositionHelper.test.ts
index e29111e18..94b5841af 100644
--- a/src/browser/input/CompositionHelper.test.ts
+++ b/src/browser/input/CompositionHelper.test.ts
@@ -233,5 +233,31 @@ describe('CompositionHelper', () => {
         }, 0);
       }, 0);
     });
+
+    it('Should insert middle composition and subsequent input without appending existing trailing text', (done) => {
+      textarea.value = '一二';
+      // screenReaderMode keeps textarea content/selection for assistive technologies (eg. screen
+      // readers), so the caret can be moved within the textarea (eg. via arrow keys) before
+      // starting composition.
+      textarea.selectionStart = 1;
+      textarea.selectionEnd = 1;
+
+      compositionHelper.compositionstart();
+      compositionHelper.compositionupdate({ data: '一' });
+      textarea.value = '一一二';
+      // After the composed text is inserted, the caret typically moves to after it.
+      textarea.selectionStart = 2;
+      textarea.selectionEnd = 2;
+
+      setTimeout(() => { // wait for any textarea updates
+        compositionHelper.compositionend();
+        // Second character '1' (a non-composition character)
+        textarea.value = '一一1二';
+        setTimeout(() => { // wait for any textarea updates
+          assert.equal(handledText, '一1');
+          done();
+        }, 0);
+      }, 0);
+    });
   });
 });
diff --git a/src/browser/input/CompositionHelper.ts b/src/browser/input/CompositionHelper.ts
index 0f4ef7789..283874139 100644
--- a/src/browser/input/CompositionHelper.ts
+++ b/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 +72,13 @@ export class CompositionHelper {
    */
   public compositionstart(): void {
     this._isComposing = true;
-    this._compositionPosition.start = this._textarea.value.length;
+    // It's important to use the selection here instead of textarea length to avoid conflicts with
+    // screen reader mode
+    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');
@@ -81,7 +94,8 @@ export class CompositionHelper {
     this._compositionView.textContent = `\u200E${ev.data}\u200E`;
     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);
   }
 
@@ -148,6 +162,7 @@ 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
@@ -171,10 +186,14 @@ 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);