WebUI Tab Strip: Move direction calculation to animate function

The checks that are necessary to determine if a TabElement moved towards
the start or the end of the tab strip are the same for TabGroupElements,
so this CL prepares the animation for TabGroupElement by moving these
checks to the animation functions.

Bug: 1082344
Change-Id: I9bd9322948c40c61e9324d74c6c0dfba41ea9551
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250950
Commit-Queue: John Lee <johntlee@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779955}

chrome/browser/resources/tab_strip/tab_list.js

@@ -50,15 +50,33 @@ const LayoutVariable = {
 /**
  * Animates a series of elements to indicate that tabs have moved position.
  * @param {!Element} movedElement
- * @param {?Element} elementsToAnimateStart
- * @param {?Element} elementsToAnimateEnd
- * @param {number} direction, +1 if moving towards the end of tab strip,
- *     -1 if moving left
- */
-function animateTabElementMoved(
-    movedElement, elementsToAnimateStart, elementsToAnimateEnd, direction) {
-  if (isRTL()) {
-    direction *= -1;
+ * @param {?Element} prevSibling, the previousElementSibling before element
+ *     was moved
+ * @param {?Element} nextSibling, the nextElementSibling before element was
+ *     moved
+ */
+function animateElementMoved(movedElement, prevSibling, nextSibling) {
+  let horizontalDirection = isRTL() ? -1 : 1;
+
+  let elementsToAnimateStart;
+  let elementsToAnimateEnd;
+  if (nextSibling &&
+      movedElement.compareDocumentPosition(nextSibling) &
+          Node.DOCUMENT_POSITION_PRECEDING) {
+    // Element moved towards end of tab strip.
+    elementsToAnimateStart = nextSibling;
+    elementsToAnimateEnd = movedElement.previousElementSibling;
+  } else if (
+      prevSibling &&
+      movedElement.compareDocumentPosition(prevSibling) &
+          Node.DOCUMENT_POSITION_FOLLOWING) {
+    // Element moved towards start of tab strip.
+    elementsToAnimateStart = movedElement.nextElementSibling;
+    elementsToAnimateEnd = prevSibling;
+    horizontalDirection *= -1;
+  } else {
+    // Element did not move.
+    return;
   }
 
   let elementToAnimate = elementsToAnimateStart;
@@ -68,27 +86,27 @@ function animateTabElementMoved(
   // elementsToAnimateEnd and animate each of them.
   while (elementToAnimate && elementsToAnimateEnd &&
          elementToAnimate !== elementsToAnimateEnd.nextElementSibling) {
-    slideElement(elementToAnimate, direction);
+    slideElement(elementToAnimate, horizontalDirection);
     elementToAnimate = elementToAnimate.nextElementSibling;
     numOfTabs++;
   }
 
   // Animate the moved TabElement itself the total number of tabs that it
   // has been moved across.
-  slideElement(movedElement, -1 * direction * numOfTabs);
+  slideElement(movedElement, -1 * horizontalDirection * numOfTabs);
 }
 
 /**
- * Animates the horizontal slide of an element across the tab strip.
+ * Animates the slide of an element across the tab strip.
  * @param {!Element} element
- * @param {number} scale
+ * @param {number} horizontalScale
  */
-function slideElement(element, scale) {
+function slideElement(element, horizontalScale) {
   element.isValidDragOverTarget = false;
   const animation = element.animate(
       [
         {
-          transform: 'translateX(calc(' + scale + ' ' +
+          transform: 'translateX(calc(' + horizontalScale + ' ' +
               '* (var(--tabstrip-tab-width) + var(--tabstrip-tab-spacing))))',
         },
         {transform: 'translateX(0)'},
@@ -707,20 +725,8 @@ export class TabListElement extends CustomElement {
     this.updateTabElementDomPosition_(element, index, pinned, groupId);
 
     if (shouldAnimate) {
-      if (initialDomNextSibling &&
-          element.compareDocumentPosition(initialDomNextSibling) &
-              Node.DOCUMENT_POSITION_PRECEDING) {
-        // Element has moved right.
-        animateTabElementMoved(
-            element, initialDomNextSibling, element.previousElementSibling, 1);
-      } else if (
-          initialDomPrevSibling &&
-          element.compareDocumentPosition(initialDomPrevSibling) &
-              Node.DOCUMENT_POSITION_FOLLOWING) {
-        // Element has moved left.
-        animateTabElementMoved(
-            element, element.nextElementSibling, initialDomPrevSibling, -1);
-      }
+      animateElementMoved(
+          element, initialDomPrevSibling, initialDomNextSibling);
     }
 
     if (isInserting) {