Rich Text: Avoid activeElement focus call (#20594)

* Rich Text: Avoid activeElement focus call

* Rich Text: Restore focus, accounting for null activeElement

* Rich Text: Verify activeElement instanceof HTMLElement

packages/rich-text/src/to-dom.js

@@ -302,5 +302,24 @@ export function applySelection( { startPath, endPath }, current ) {
 	}
 
 	selection.addRange( range );
-	activeElement.focus();
+
+	// This function is not intended to cause a shift in focus. Since the above
+	// selection manipulations may shift focus, ensure that focus is restored to
+	// its previous state. `activeElement` can be `null` or the body element if
+	// there is no focus, which is accounted for here in the explicit `blur` to
+	// restore to a state of non-focus.
+	if ( activeElement !== document.activeElement ) {
+		// The `instanceof` checks protect against edge cases where the focused
+		// element is not of the interface HTMLElement (does not have a `focus`
+		// or `blur` property).
+		//
+		// See: https://github.com/Microsoft/TypeScript/issues/5901#issuecomment-431649653
+		if ( activeElement ) {
+			if ( activeElement instanceof window.HTMLElement ) {
+				activeElement.focus();
+			}
+		} else if ( document.activeElement instanceof window.HTMLElement ) {
+			document.activeElement.blur();
+		}
+	}
 }