Add top to xterm.js' textarea

Fixes #12038

src/vs/workbench/parts/terminal/electron-browser/media/xterm.css

@@ -38,6 +38,7 @@
 	 * Move textarea out of the screen to the far left, so that the cursor is not visible.
 	 */
 	left: -9999em;
+	top: -9999em;
 	opacity: 0;
 	width: 0;
 	height: 0;