Remember scroll position during a session on MR tabs

app/assets/javascripts/merge_request_tabs.js

@@ -64,6 +64,8 @@ import syntaxHighlight from './syntax_highlight';
 //   </div>
 //
 
+// <100ms is typically indistinguishable from "instant" for users, but allows for re-rendering
+const FAST_DELAY_FOR_RERENDER = 75;
 // Store the `location` object, allowing for easier stubbing in tests
 let { location } = window;
 
@@ -83,6 +85,8 @@ export default class MergeRequestTabs {
     this.peek = document.getElementById('js-peek');
     this.paddingTop = 16;
 
+    this.scrollPositions = {};
+
     this.commitsTab = document.querySelector('.tab-content .commits.tab-pane');
 
     this.currentTab = null;
@@ -136,11 +140,30 @@ export default class MergeRequestTabs {
     }
   }
 
+  storeScroll() {
+    if (this.currentTab) {
+      this.scrollPositions[this.currentTab] = document.documentElement.scrollTop;
+    }
+  }
+  recallScroll(action) {
+    const storedPosition = this.scrollPositions[action];
+
+    setTimeout(() => {
+      window.scrollTo({
+        top: storedPosition && storedPosition > 0 ? storedPosition : 0,
+        left: 0,
+        behavior: 'auto',
+      });
+    }, FAST_DELAY_FOR_RERENDER);
+  }
+
   clickTab(e) {
     if (e.currentTarget) {
       e.stopImmediatePropagation();
       e.preventDefault();
 
+      this.storeScroll();
+
       const { action } = e.currentTarget.dataset || {};
 
       if (isMetaClick(e)) {
@@ -210,8 +233,14 @@ export default class MergeRequestTabs {
         this.resetViewContainer();
         this.mountPipelinesView();
       } else {
-        this.mergeRequestTabPanes.querySelector('#notes').style.display = 'block';
-        this.mergeRequestTabs.querySelector('.notes-tab').classList.add('active');
+        const notesTab = this.mergeRequestTabs.querySelector('.notes-tab');
+        const notesPane = this.mergeRequestTabPanes.querySelector('#notes');
+        if (notesPane) {
+          notesPane.style.display = 'block';
+        }
+        if (notesTab) {
+          notesTab.classList.add('active');
+        }
 
         if (bp.getBreakpointSize() !== 'xs') {
           this.expandView();
@@ -221,6 +250,8 @@ export default class MergeRequestTabs {
       }
 
       $('.detail-page-description').renderGFM();
+
+      this.recallScroll(action);
     } else if (action === this.currentAction) {
       // ContentTop is used to handle anything at the top of the page before the main content
       const mainContentContainer = document.querySelector('.content-wrapper');

spec/frontend/merge_request_tabs_spec.js

@@ -34,6 +34,44 @@ describe('MergeRequestTabs', () => {
     gl.mrWidget = {};
   });
 
+  describe('clickTab', () => {
+    let params;
+
+    beforeEach(() => {
+      document.documentElement.scrollTop = 100;
+
+      params = {
+        metaKey: false,
+        ctrlKey: false,
+        which: 1,
+        stopImmediatePropagation() {},
+        preventDefault() {},
+        currentTarget: {
+          getAttribute(attr) {
+            return attr === 'href' ? 'a/tab/url' : null;
+          },
+        },
+      };
+    });
+
+    it("stores the current scroll position if there's an active tab", () => {
+      testContext.class.currentTab = 'someTab';
+
+      testContext.class.clickTab(params);
+
+      expect(testContext.class.scrollPositions.someTab).toBe(100);
+    });
+
+    it("doesn't store a scroll position if there's no active tab", () => {
+      // this happens on first load, and we just don't want to store empty values in the `null` property
+      testContext.class.currentTab = null;
+
+      testContext.class.clickTab(params);
+
+      expect(testContext.class.scrollPositions).toEqual({});
+    });
+  });
+
   describe('opensInNewTab', () => {
     const windowTarget = '_blank';
     let clickTabParams;
@@ -258,6 +296,7 @@ describe('MergeRequestTabs', () => {
     beforeEach(() => {
       jest.spyOn(mainContent, 'getBoundingClientRect').mockReturnValue({ top: 10 });
       jest.spyOn(tabContent, 'getBoundingClientRect').mockReturnValue({ top: 100 });
+      jest.spyOn(window, 'scrollTo').mockImplementation(() => {});
       jest.spyOn(document, 'querySelector').mockImplementation((selector) => {
         return selector === '.content-wrapper' ? mainContent : tabContent;
       });
@@ -267,8 +306,6 @@ describe('MergeRequestTabs', () => {
     it('calls window scrollTo with options if document has scrollBehavior', () => {
       document.documentElement.style.scrollBehavior = '';
 
-      jest.spyOn(window, 'scrollTo').mockImplementation(() => {});
-
       testContext.class.tabShown('commits', 'foobar');
 
       expect(window.scrollTo.mock.calls[0][0]).toEqual({ top: 39, behavior: 'smooth' });
@@ -276,11 +313,50 @@ describe('MergeRequestTabs', () => {
 
     it('calls window scrollTo with two args if document does not have scrollBehavior', () => {
       jest.spyOn(document.documentElement, 'style', 'get').mockReturnValue({});
-      jest.spyOn(window, 'scrollTo').mockImplementation(() => {});
 
       testContext.class.tabShown('commits', 'foobar');
 
       expect(window.scrollTo.mock.calls[0]).toEqual([0, 39]);
     });
+
+    describe('when switching tabs', () => {
+      const SCROLL_TOP = 100;
+
+      beforeAll(() => {
+        jest.useFakeTimers();
+      });
+
+      beforeEach(() => {
+        jest.spyOn(window, 'scrollTo').mockImplementation(() => {});
+        testContext.class.mergeRequestTabs = document.createElement('div');
+        testContext.class.mergeRequestTabPanes = document.createElement('div');
+        testContext.class.currentTab = 'tab';
+        testContext.class.scrollPositions = { newTab: SCROLL_TOP };
+      });
+
+      afterAll(() => {
+        jest.useRealTimers();
+      });
+
+      it('scrolls to the stored position, if one is stored', () => {
+        testContext.class.tabShown('newTab');
+
+        jest.advanceTimersByTime(250);
+
+        expect(window.scrollTo.mock.calls[0][0]).toEqual({
+          top: SCROLL_TOP,
+          left: 0,
+          behavior: 'auto',
+        });
+      });
+
+      it('scrolls to 0, if no position is stored', () => {
+        testContext.class.tabShown('unknownTab');
+
+        jest.advanceTimersByTime(250);
+
+        expect(window.scrollTo.mock.calls[0][0]).toEqual({ top: 0, left: 0, behavior: 'auto' });
+      });
+    });
   });
 });