review: fix scroll tracking for element-level scrolls

Author: archandatta

server/lib/cdpmonitor/interaction.js

@@ -27,24 +27,38 @@
     }));
   }, true);
 
+  function scrollPos(target) {
+    if (target === document || target === document.documentElement) {
+      return {x: window.scrollX, y: window.scrollY};
+    }
+    return {x: target.scrollLeft || 0, y: target.scrollTop || 0};
+  }
+
   var scrollTimer = null;
-  var scrollStart = {x: window.scrollX, y: window.scrollY};
+  var scrollStart = null;
+  var scrollTarget = null;
   document.addEventListener('scroll', function(e) {
-    var fromX = scrollStart.x, fromY = scrollStart.y;
     var target = e.target;
+    // If target changed mid-scroll, reset tracking for the new target.
+    if (scrollTarget !== target) {
+      scrollStart = scrollPos(target);
+      scrollTarget = target;
+    }
+    var fromX = scrollStart.x, fromY = scrollStart.y;
     var s = target === document ? 'document' : sel(target);
     if (scrollTimer) clearTimeout(scrollTimer);
     scrollTimer = setTimeout(function() {
-      var toX = window.scrollX, toY = window.scrollY;
-      if (Math.abs(toX - fromX) > 5 || Math.abs(toY - fromY) > 5) {
+      var pos = scrollPos(target);
+      if (Math.abs(pos.x - fromX) > 5 || Math.abs(pos.y - fromY) > 5) {
         send(JSON.stringify({
           type: 'scroll_settled',
           from_x: fromX, from_y: fromY,
-          to_x: toX, to_y: toY,
+          to_x: pos.x, to_y: pos.y,
           target_selector: s
         }));
       }
-      scrollStart = {x: toX, y: toY};
+      scrollStart = {x: pos.x, y: pos.y};
+      scrollTarget = null;
     }, 300);
   }, true);
 })();