Adds container around merge request header

As a follow-up to making the diffs be fluid layout by default
this makes the header (including the tabs) always keep the users
preferred layout mode.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/343042

app/assets/javascripts/diffs/components/app.vue

@@ -392,8 +392,6 @@ export default {
     diffsApp.instrument();
   },
   created() {
-    this.mergeRequestContainers = document.querySelectorAll('.merge-request-container');
-
     this.adjustView();
     this.subscribeToEvents();
 
@@ -521,13 +519,6 @@ export default {
       } else {
         this.removeEventListeners();
       }
-
-      if (!this.isFluidLayout && this.glFeatures.mrChangesFluidLayout) {
-        this.mergeRequestContainers.forEach((el) => {
-          el.classList.toggle('limit-container-width', !this.shouldShow);
-          el.classList.toggle('container-limited', !this.shouldShow);
-        });
-      }
     },
     setEventListeners() {
       Mousetrap.bind(keysFor(MR_PREVIOUS_FILE_IN_DIFF), () => this.jumpToFile(-1));

spec/frontend/diffs/components/app_spec.js

@@ -702,23 +702,4 @@ describe('diffs/components/app', () => {
       );
     });
   });
-
-  describe('fluid layout', () => {
-    beforeEach(() => {
-      setFixtures(
-        '<div><div class="merge-request-container limit-container-width container-limited"></div></div>',
-      );
-    });
-
-    it('removes limited container classes when on diffs tab', () => {
-      createComponent({ isFluidLayout: false, shouldShow: true }, () => {}, {
-        glFeatures: { mrChangesFluidLayout: true },
-      });
-
-      const containerClassList = document.querySelector('.merge-request-container').classList;
-
-      expect(containerClassList).not.toContain('container-limited');
-      expect(containerClassList).not.toContain('limit-container-width');
-    });
-  });
 });