Set _loading flag in diff-view while awaiting API

Previously, PolyGerrit relied on the default value of the loading flag
in the diff view to show/hide the appropriate diff panels (or loading
message). Because of this, stale information is displayed when a user
navigates between different files within the same diff view.

With this change, the `_loading` flag is set before the major API calls
are made, and the loading message is made more obvious.

Bug: Issue 7381
Change-Id: Id70d981a2c72fef9f44a32c5463dc43c3ce41cf6
(cherry picked from commit 29b2ab4a69)

polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html

@@ -124,8 +124,11 @@ limitations under the License.
         text-decoration: none;
       }
       .loading {
-        padding: 0 var(--default-horizontal-margin) 1em;
+        color: #777;
-        color: #666;
+        font-size: 2em;
+        height: 100%;
+        padding: 1em var(--default-horizontal-margin);
+        text-align: center;
       }
       .subHeader {
         flex-wrap: wrap;

polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js

@@ -540,6 +540,7 @@
 
       promises.push(this._getChangeEdit(this._changeNum));
 
+      this._loading = true;
       Promise.all(promises).then(r => {
         const edit = r[4];
         if (edit) {

polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html

@@ -100,23 +100,27 @@ limitations under the License.
           '10', PARENT), 'Should navigate to /c/42/10/wheatley.md');
       element._path = 'wheatley.md';
       assert.equal(element.changeViewState.selectedFileIndex, 2);
+      assert.isTrue(element._loading);
 
       MockInteractions.pressAndReleaseKeyOn(element, 219, null, '[');
       assert(diffNavStub.lastCall.calledWith(element._change, 'glados.txt',
           '10', PARENT), 'Should navigate to /c/42/10/glados.txt');
       element._path = 'glados.txt';
       assert.equal(element.changeViewState.selectedFileIndex, 1);
+      assert.isTrue(element._loading);
 
       MockInteractions.pressAndReleaseKeyOn(element, 219, null, '[');
       assert(diffNavStub.lastCall.calledWith(element._change, 'chell.go', '10',
           PARENT), 'Should navigate to /c/42/10/chell.go');
       element._path = 'chell.go';
       assert.equal(element.changeViewState.selectedFileIndex, 0);
+      assert.isTrue(element._loading);
 
       MockInteractions.pressAndReleaseKeyOn(element, 219, null, '[');
       assert(changeNavStub.lastCall.calledWith(element._change),
           'Should navigate to /c/42/');
       assert.equal(element.changeViewState.selectedFileIndex, 0);
+      assert.isTrue(element._loading);
 
       const showPrefsStub =
           sandbox.stub(element.$.diffPreferences.$.prefsOverlay, 'open',
@@ -190,24 +194,28 @@ limitations under the License.
           '5'), 'Should navigate to /c/42/5..10');
 
       MockInteractions.pressAndReleaseKeyOn(element, 221, null, ']');
+      assert.isTrue(element._loading);
       assert(diffNavStub.lastCall.calledWithExactly(element._change,
           'wheatley.md', '10', '5'),
           'Should navigate to /c/42/5..10/wheatley.md');
       element._path = 'wheatley.md';
 
       MockInteractions.pressAndReleaseKeyOn(element, 219, null, '[');
+      assert.isTrue(element._loading);
       assert(diffNavStub.lastCall.calledWithExactly(element._change,
           'glados.txt', '10', '5'),
           'Should navigate to /c/42/5..10/glados.txt');
       element._path = 'glados.txt';
 
       MockInteractions.pressAndReleaseKeyOn(element, 219, null, '[');
+      assert.isTrue(element._loading);
       assert(diffNavStub.lastCall.calledWithExactly(element._change, 'chell.go',
           '10', '5'),
           'Should navigate to /c/42/5..10/chell.go');
       element._path = 'chell.go';
 
       MockInteractions.pressAndReleaseKeyOn(element, 219, null, '[');
+      assert.isTrue(element._loading);
       assert(changeNavStub.lastCall.calledWithExactly(element._change, '10',
           '5'),
           'Should navigate to /c/42/5..10');