fix: patch onSidebarDragStart to strip trailing whitespace nodes and prevent DOM injection errors

static/src/js/sign_backend_patch.js

@@ -116,5 +116,30 @@ patch(SignTemplateBody.prototype, {
             }
         }
         return [updatedSignItems, Id2UpdatedItem];
+    },
+
+    /**
+     * FIX: Odoo Core Bug where trailing whitespace on custom items causes target.lastChild to be a Text Node.
+     * This patch intercepts the insertAdjacentHTML during onSidebarDragStart to strip trailing whitespace nodes immediately,
+     * without modifying Odoo Enterprise source code.
+     */
+    onSidebarDragStart(e) {
+        const firstPage = this.root.querySelector('.page[data-page-number="1"]');
+        if (firstPage && firstPage.insertAdjacentHTML) {
+            const originalInsert = firstPage.insertAdjacentHTML;
+            firstPage.insertAdjacentHTML = function(position, text) {
+                originalInsert.call(this, position, text.trim());
+                // Remove trailing whitespace nodes injected by QWeb
+                while (this.lastChild && this.lastChild.nodeType !== 1) { // 1 == ELEMENT_NODE
+                    this.lastChild.remove();
+                }
+            };
+            try {
+                return super.onSidebarDragStart(...arguments);
+            } finally {
+                firstPage.insertAdjacentHTML = originalInsert;
+            }
+        }
+        return super.onSidebarDragStart(...arguments);
     }
 });