Item pane header: Use focused getter instead of :focus-within

:focus-within doesn't match when the window is inactive but the field
is focused.

Addresses #4101, except that focusing the new item's title field and
pressing Escape will clear the field (without modifying the item).

chrome/content/zotero/elements/itemPaneHeader.js

@@ -179,7 +179,7 @@
 		}
 		
 		async blurOpenField() {
-			if (this.titleField?.matches(':focus-within')) {
+			if (this.titleField.focused) {
 				this.titleField.blur();
 				await this.save();
 			}