diff --git a/modules/browser.js b/modules/browser.js
index cd780c64..a62cba8d 100644
--- a/modules/browser.js
+++ b/modules/browser.js
@@ -2566,6 +2566,12 @@ TreeStyleTabBrowser.prototype = {
 			this.updateTabCollapsed(tab, true, true);
 			let self = this;
 			this.updateTabCollapsed(tab, false, this.windowService.restoringTree, function() {
+				/**
+				 * When the system is too slow, the animation can start after
+				 * smooth scrolling is finished. However, the smooth scrolling
+				 * should be started together with the start of the animation
+				 * effect.
+				 */
 				self.scrollToNewTab(tab);
 			});
 		}