If file is unchanged, start scrolled to top

The previous code has a bug where when you navigate to the next file
using keyboard shortcuts, and the next file does not have changes, the
cursor checks all regions of interest (comments and such) and because
they are not diffs, moves past them to the bottom of the diff.

This change allows us to specify the intention to go back to top
whenever we get out of range to prevent this.

Change-Id: I7d00a43115ca0f926fb7a7f1b93084ff5ef0e334

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

@@ -136,11 +136,11 @@
       }
     },
 
-    moveToNextChunk() {
+    moveToNextChunk(opt_clipToTop) {
       this.$.cursorManager.next(this._isFirstRowOfChunk.bind(this),
           target => {
             return target.parentNode.scrollHeight;
-          });
+          }, opt_clipToTop);
       this._fixSide();
     },
 
@@ -200,7 +200,7 @@
 
     moveToFirstChunk() {
       this.$.cursorManager.moveToStart();
-      this.moveToNextChunk();
+      this.moveToNextChunk(true);
     },
 
     reInitCursor() {

polygerrit-ui/app/elements/shared/gr-cursor-manager/gr-cursor-manager.js

@@ -90,8 +90,21 @@
       this.unsetCursor();
     },
 
-    next(opt_condition, opt_getTargetHeight) {
-      this._moveCursor(1, opt_condition, opt_getTargetHeight);
+    /**
+     * Move the cursor forward. Clipped to the ends of the stop list.
+     * @param {!Function=} opt_condition Optional stop condition. If a condition
+     *    is passed the cursor will continue to move in the specified direction
+     *    until the condition is met.
+     * @param {!Function=} opt_getTargetHeight Optional function to calculate the
+     *    height of the target's 'section'. The height of the target itself is
+     *    sometimes different, used by the diff cursor.
+     * @param {boolean=} opt_clipToTop When none of the next indices match, move
+     *     back to first instead of to last.
+     * @private
+     */
+
+    next(opt_condition, opt_getTargetHeight, opt_clipToTop) {
+      this._moveCursor(1, opt_condition, opt_getTargetHeight, opt_clipToTop);
     },
 
     previous(opt_condition) {
@@ -145,8 +158,8 @@
     },
 
     /**
-     * Move the cursor forward or backward by delta. Noop if moving past either
-     * end of the stop list.
+     * Move the cursor forward or backward by delta. Clipped to the beginning or
+     * end of stop list.
      * @param {number} delta either -1 or 1.
      * @param {!Function=} opt_condition Optional stop condition. If a condition
      *    is passed the cursor will continue to move in the specified direction
@@ -154,9 +167,11 @@
      * @param {!Function=} opt_getTargetHeight Optional function to calculate the
      *    height of the target's 'section'. The height of the target itself is
      *    sometimes different, used by the diff cursor.
+     * @param {boolean=} opt_clipToTop When none of the next indices match, move
+     *     back to first instead of to last.
      * @private
      */
-    _moveCursor(delta, opt_condition, opt_getTargetHeight) {
+    _moveCursor(delta, opt_condition, opt_getTargetHeight, opt_clipToTop) {
       if (!this.stops.length) {
         this.unsetCursor();
         return;
@@ -164,7 +179,7 @@
 
       this._unDecorateTarget();
 
-      const newIndex = this._getNextindex(delta, opt_condition);
+      const newIndex = this._getNextindex(delta, opt_condition, opt_clipToTop);
 
       let newTarget = null;
       if (newIndex !== -1) {
@@ -203,10 +218,12 @@
      * Get the next stop index indicated by the delta direction.
      * @param {number} delta either -1 or 1.
      * @param {!Function=} opt_condition Optional stop condition.
+     * @param {boolean=} opt_clipToTop When none of the next indices match, move
+     *     back to first instead of to last.
      * @return {number} the new index.
      * @private
      */
-    _getNextindex(delta, opt_condition) {
+    _getNextindex(delta, opt_condition, opt_clipToTop) {
       if (!this.stops.length || this.index === -1) {
         return -1;
       }
@@ -222,10 +239,10 @@
 
       // If we failed to satisfy the condition:
       if (opt_condition && !opt_condition(this.stops[newIndex])) {
-        if (delta > 0) {
-          return this.stops.length - 1;
-        } else if (delta < 0) {
+        if (delta < 0 || opt_clipToTop) {
           return 0;
+        } else if (delta > 0) {
+          return this.stops.length - 1;
         }
         return this.index;
       }