Make reinitializing cursor explicit

Initializing the cursor potentially does disruptive things like stealing
the cursor and scrolling the page - the behavior to do this on every
render should not be baked into gr-diff(-cursor). Instead, it is better
if pages that want this behavior trigger it explicitly.

The list view already does that (by calling `handleDiffUpdate()`)
when its done rendering all diffs.

This change
* makes gr-diff-view ("the file page") call reInitCursor explicitly when
  done  rendering
* removes that behavior from the reusable gr-diff-cursor

One positive side-effect is that on the list view, we have less
duplicate cursor initializations (after every diff renders, and then
again when they are all done rendering).

Change-Id: I04602bec25af753ec923914db4b5be8b33a45449

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

@@ -309,7 +309,7 @@ class GrDiffCursor extends mixinBehaviors([Gerrit.FireBehavior],
   }
 
   _handleDiffRenderContent() {
-    this.handleDiffUpdate();
+    this._updateStops();
     // When done rendering, turn focus on move and automatic scrolling back on
     this._focusOnMove = true;
     this._preventAutoScrollOnManualScroll = false;

polygerrit-ui/app/elements/diff/gr-diff-cursor/gr-diff-cursor_test.html

@@ -120,8 +120,9 @@ suite('gr-diff-cursor tests', () => {
   });
 
   test('cursor scroll behavior', () => {
-    cursorElement._handleDiffRenderStart();
     assert.equal(cursorElement._scrollBehavior, 'keep-visible');
+
+    cursorElement._handleDiffRenderStart();
     assert.isTrue(cursorElement._focusOnMove);
 
     cursorElement._handleWindowScroll();
@@ -129,8 +130,10 @@ suite('gr-diff-cursor tests', () => {
     assert.isFalse(cursorElement._focusOnMove);
 
     cursorElement._handleDiffRenderContent();
-    assert.equal(cursorElement._scrollBehavior, 'keep-visible');
     assert.isTrue(cursorElement._focusOnMove);
+
+    cursorElement.reInitCursor();
+    assert.equal(cursorElement._scrollBehavior, 'keep-visible');
   });
 
   suite('unified diff', () => {
@@ -238,6 +241,7 @@ suite('gr-diff-cursor tests', () => {
 
     function renderHandler() {
       diffElement.removeEventListener('render', renderHandler);
+      cursorElement.reInitCursor();
       assert.isFalse(moveToNumStub.called);
       assert.isTrue(moveToChunkStub.called);
       assert.equal(scrollBehaviorDuringMove, 'never');
@@ -255,6 +259,7 @@ suite('gr-diff-cursor tests', () => {
     const moveToChunkStub = sandbox.stub(cursorElement, 'moveToFirstChunk');
     function renderHandler() {
       diffElement.removeEventListener('render', renderHandler);
+      cursorElement.reInitCursor();
       assert.isFalse(moveToChunkStub.called);
       assert.isTrue(moveToNumStub.called);
       assert.equal(moveToNumStub.lastCall.args[0], 10);
@@ -382,11 +387,11 @@ suite('gr-diff-cursor tests', () => {
   });
 
   test('expand context updates stops', done => {
-    sandbox.spy(cursorElement, 'handleDiffUpdate');
+    sandbox.spy(cursorElement, '_updateStops');
     MockInteractions.tap(diffElement.shadowRoot
         .querySelector('.showContext'));
     flush(() => {
-      assert.isTrue(cursorElement.handleDiffUpdate.called);
+      assert.isTrue(cursorElement._updateStops.called);
       done();
     });
   });

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

@@ -314,6 +314,12 @@ class GrDiffView extends mixinBehaviors( [
     this.addEventListener('open-fix-preview',
         this._onOpenFixPreview.bind(this));
     this.$.cursor.push('diffs', this.$.diffHost);
+
+    const onRender = () => {
+      this.$.diffHost.removeEventListener('render', onRender);
+      this.$.cursor.reInitCursor();
+    };
+    this.$.diffHost.addEventListener('render', onRender);
   }
 
   _getLoggedIn() {