Simplify GameState legacy HTML string support

Move the getInnerHTML fallback to the default implementation of
getContentLayout; this allows for code reuse in TextualGameState too.

src/js/core/game_state.ts

@@ -200,10 +200,12 @@ export class GameState {
 
     /**
      * Should return the element(s) to be displayed in the state.
-     * If null, {@link getInnerHTML} will be used instead.
+     * If not overridden, {@link getInnerHTML} will be used to provide the layout.
      */
     protected getContentLayout(): Node {
-        return null;
+        const template = document.createElement("template");
+        template.innerHTML = this.getInnerHTML();
+        return template.content;
     }
 
     /**
@@ -320,9 +322,6 @@ export class GameState {
      */
     internalGetWrappedContent(): Node {
         const elements = this.getContentLayout();
-        if (elements instanceof Node) {
-            return elements;
-        }
 
         if (Array.isArray(elements)) {
             const fragment = document.createDocumentFragment();
@@ -330,10 +329,7 @@ export class GameState {
             return fragment;
         }
 
-        // Fall back to deprecated HTML strings solution
+        return elements;
-        const template = document.createElement("template");
-        template.innerHTML = this.getInnerHTML();
-        return template.content;
     }
 
     /**

src/js/core/textual_game_state.tsx

@@ -30,6 +30,11 @@ export abstract class TextualGameState extends GameState {
         return "";
     }
 
+    /**
+     * Should return the element(s) to be displayed in the state.
+     * If not overridden, a default layout consisting of a back button,
+     * title, and content returned by {@link getInitialContent}.
+     */
     protected override getContentLayout(): Node {
         let content = this.getInitialContent();
 
@@ -44,9 +49,7 @@ export abstract class TextualGameState extends GameState {
                 `;
             }
 
-            const template = document.createElement("template");
+            content = super.getContentLayout();
-            template.innerHTML = html;
-            content = template.content;
         }
 
         return (