Cancel keydown events when not in screenReaderMode

This caused issues with embedder keybinding systems firing when they
shouldn't be. The fix is to only allow it in screenReaderMode as a
compromise so keys are echoed.

src/Terminal.ts

@@ -1568,10 +1568,19 @@ export class Terminal extends Disposable implements ITerminal, IDisposable, IInp
       this.textarea.value = '';
     }
 
-    this._keyDownHandled = true;
     this._onKey.fire({ key: result.key, domEvent: event });
     this.showCursor();
     this._coreService.triggerDataEvent(result.key, true);
+
+    // Cancel events when not in screen reader mode so events don't get bubbled up and handled by
+    // other listeners. When screen reader mode is enabled, this could cause issues if the event
+    // is handled at a higher level, this is a compromise in order to echo keys to the screen
+    // reader.
+    if (!this.optionsService.options.screenReaderMode) {
+      return this.cancel(event, true);
+    }
+
+    this._keyDownHandled = true;
   }
 
   private _isThirdLevelShift(browser: IBrowser, ev: IKeyboardEvent): boolean {