Fix EPUB not getting keyboard focus on tab open

(Why this happens remains a mystery, but focusing the outer window first seems
 to fix it.)

Fixes zotero/zotero#3325

src/dom/common/dom-view.tsx

@@ -948,6 +948,8 @@ abstract class DOMView<State extends DOMViewState, Data> {
 	// ***
 
 	focus() {
+		// The iframe sometimes won't correctly get focus unless we focus the outer window first
+		window.focus();
 		this._iframe.focus();
 	}