Fix re-use of extensions between instances

Changelog: fixed

app/assets/javascripts/editor/source_editor_instance.js

@@ -50,6 +50,10 @@ const utils = {
     }
     return extensionsStore.get(extensionName);
   },
+
+  hasFullApiRegistered: (targetMethods, newMethods) => {
+    return newMethods.find((fn) => !targetMethods.includes(fn)) === undefined;
+  },
 };
 
 /** Class representing a Source Editor Instance */
@@ -132,7 +136,9 @@ export default class EditorInstance {
     const existingExt = utils.getStoredExtension(extensionsStore, definition.extensionName);
     if (existingExt) {
       if (isEqual(extension.setupOptions, existingExt.setupOptions)) {
-        return existingExt;
+        if (utils.hasFullApiRegistered(this.extensionsAPI, Object.keys(existingExt.api))) {
+          return existingExt;
+        }
       }
       this.unuseExtension(extensionsStore, existingExt);
     }

spec/frontend/editor/source_editor_instance_spec.js

@@ -277,6 +277,17 @@ describe('Source Editor Instance', () => {
           expect(extensionStore.set).toHaveBeenCalledWith('SEClassExtension', extension1);
         });
 
+        it('correctly registers methods from the existing extension on an instance', () => {
+          const seInstance2 = new SourceEditorInstance({}, extensionStore);
+          seInstance.use({ definition: SEClassExtension });
+          const val1 = seInstance.classExtMethod();
+
+          seInstance2.use({ definition: SEClassExtension });
+
+          expect(seInstance2.classExtMethod).toBeDefined();
+          expect(seInstance2.classExtMethod()).toBe(val1); // from helpers.js we know classExtMethod()returns a string. Hence `toBe`
+        });
+
         it.each`
           desc                 | currentSetupOptions | newSetupOptions    | expectedCallTimes
           ${'updates'}         | ${undefined}        | ${defSetupOptions} | ${2}