Add a custom mixin that allows doing complex structural position while maintaining partial rerenders

module/Apps/mixins/LaidOutAppMixin.mjs

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+/**
+ * This mixin makes it so that we can provide a specific layout template without
+ * needing to reference each of the inner areas via a partial embedded in the root,
+ * enabling partial re-renders for parts of the sheet without losing advanced
+ * layout capabilities.
+ *
+ * @param {ReturnType<HandlebarsApp>} HandlebarsApp The mixin'd class from HAM to further mix
+ */
+export function LaidOutAppMixin(HandlebarsApp) {
+	class LaidOutApp extends HandlebarsApp {
+		#partDescriptors;
+
+		/**
+		 * This caches the part descriptors into this class of the heirarchy, because
+		 * Foundry doesn't expose the partDescriptors from the HAM directly, so we
+		 * inject a heirarchy call so that we can nab the pointer that Foundry has
+		 * in the HAM so that we can also read/write it from this class.
+		 */
+		_configureRenderParts(options) {
+			const parts = super._configureRenderParts(options);
+			this.#partDescriptors = parts;
+			return parts;
+		};
+
+		/**
+		 * @override
+		 * This is essentially Foundry's HandlebarsApplicationMixin implementation,
+		 * however if an existing part for non-root elements don't get concatenated
+		 * into the DOM.
+		 */
+		_replaceHTML(result, content, options) {
+			const partInfo = this.#partDescriptors;
+			for ( const [partId, htmlElement] of Object.entries(result) ) {
+				const part = partInfo[partId];
+				const priorElement = part.root ? content : content.querySelector(`[data-application-part="${partId}"]`);
+				const state = {};
+				if ( priorElement ) {
+					this._preSyncPartState(partId, htmlElement, priorElement, state);
+					if ( part.root ) {
+						priorElement.replaceChildren(...htmlElement.children);
+					}
+					else {
+						priorElement.replaceWith(htmlElement);
+					}
+					this._syncPartState(partId, htmlElement, priorElement, state);
+				}
+				else {
+					continue;
+				};
+				this._attachPartListeners(partId, htmlElement, options);
+				this.parts[partId] = htmlElement;
+			}
+		};
+	};
+	return LaidOutApp;
+};