Prevent scroll on focus

Fixes #1981

src/Terminal.ts

@@ -343,7 +343,7 @@ export class Terminal extends EventEmitter implements ITerminal, IDisposable, II
    */
   public focus(): void {
     if (this.textarea) {
-      this.textarea.focus();
+      this.textarea.focus({ preventScroll: true });
     }
   }