From c16c8b5937d7f639a24245254497751646aa08a8 Mon Sep 17 00:00:00 2001
From: Olli Pettay <Olli.Pettay@helsinki.fi>
Date: Sat, 14 Apr 2018 15:13:23 +0300
Subject: [PATCH] Bug 1452825, use smooth scrolling also for history
 navigations, r=bz

--HG--
extra : rebase_source : 562e0123597484851db6241912b7cd7e275078d9
---
 docshell/base/nsDocShell.cpp                  |  9 ++-
 .../cssom-view/scroll-behavior-smooth.html    | 80 +++++++++++++++++++
 2 files changed, 87 insertions(+), 2 deletions(-)

diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index 6f7b56004cd6..7126171f0545 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -6159,8 +6159,13 @@ nsDocShell::SetCurScrollPosEx(int32_t aCurHorizontalPos,
   nsIScrollableFrame* sf = GetRootScrollFrame();
   NS_ENSURE_TRUE(sf, NS_ERROR_FAILURE);
 
-  sf->ScrollTo(nsPoint(aCurHorizontalPos, aCurVerticalPos),
-               nsIScrollableFrame::INSTANT);
+  nsIScrollableFrame::ScrollMode scrollMode = nsIScrollableFrame::INSTANT;
+  if (sf->GetScrollbarStyles().mScrollBehavior ==
+        NS_STYLE_SCROLL_BEHAVIOR_SMOOTH) {
+    scrollMode = nsIScrollableFrame::SMOOTH_MSD;
+  }
+
+  sf->ScrollTo(nsPoint(aCurHorizontalPos, aCurVerticalPos), scrollMode);
   return NS_OK;
 }
 
diff --git a/testing/web-platform/tests/css/cssom-view/scroll-behavior-smooth.html b/testing/web-platform/tests/css/cssom-view/scroll-behavior-smooth.html
index f144a5f61cdd..e1a7a6a16803 100644
--- a/testing/web-platform/tests/css/cssom-view/scroll-behavior-smooth.html
+++ b/testing/web-platform/tests/css/cssom-view/scroll-behavior-smooth.html
@@ -56,5 +56,85 @@
     window.scrollTo(0, 0);
   }, "BODY element scroll-behavior should not propagate to viewport");
 
+  var instantHistoryNavigationTest =
+    async_test("Instant scrolling while doing history navigation.");
+  var smoothHistoryNavigationTest =
+    async_test("Smooth scrolling while doing history navigation.");
+
+  function instant() {
+    document.documentElement.className = "";
+    document.body.className = "";
+    window.scrollTo(0, 0);
+    var p = document.createElement("pre");
+    p.textContent = new Array(1000).join("newline\n");
+    var a = document.createElement("a");
+    a.href = "#";
+    a.name = "foo";
+    a.textContent = "foo";
+    p.appendChild(a);
+    document.body.appendChild(p);
+    window.onhashchange = function() {
+      window.onhashchange = function() {
+        instantHistoryNavigationTest.step(function() {
+          assert_equals(location.hash, "", "Shouldn't be scrolled to a fragment.");
+          assert_equals(window.scrollY, 0, "Shouldn't be scrolled back to top yet.");
+        });
+        p.remove();
+        instantHistoryNavigationTest.done();
+        smooth();
+      }
+
+      instantHistoryNavigationTest.step(function() {
+        assert_equals(location.hash, "#foo", "Should be scrolled to a fragment.");
+        assert_not_equals(window.scrollY, 0, "Shouldn't be scrolled to top anymore.");
+      });
+      history.back();
+    }
+
+    instantHistoryNavigationTest.step(function() {
+      assert_equals(window.scrollY, 0, "Should be scrolled to top.");
+      assert_equals(location.hash, "", "Shouldn't be scrolled to a fragment.");
+    });
+    location.hash = "foo";
+  };
+  instant();
+
+  function smooth() {
+    document.documentElement.className = "";
+    document.body.className = "";
+    window.scrollTo(0, 0);
+    var p = document.createElement("pre");
+    p.textContent = new Array(1000).join("newline\n");
+    var a = document.createElement("a");
+    a.href = "#";
+    a.name = "bar";
+    a.textContent = "bar";
+    p.appendChild(a);
+    document.body.appendChild(p);
+    window.onhashchange = function() {
+      window.onhashchange = function() {
+        smoothHistoryNavigationTest.step(function() {
+          assert_equals(location.hash, "", "Shouldn't be scrolled to a fragment.");
+          assert_not_equals(window.scrollY, 0, "Shouldn't be scrolled back to top yet.");
+        });
+        p.remove();
+        smoothHistoryNavigationTest.done();
+      }
+
+      smoothHistoryNavigationTest.step(function() {
+        assert_equals(location.hash, "#bar", "Should be scrolled to a fragment.");
+        assert_not_equals(window.scrollY, 0, "Shouldn't be scrolled to top anymore.");
+      });
+      history.back();
+    }
+
+    smoothHistoryNavigationTest.step(function() {
+      assert_equals(window.scrollY, 0, "Should be scrolled to top.");
+      assert_equals(location.hash, "", "Shouldn't be scrolled to a fragment.");
+    });
+    location.hash = "bar";
+    document.documentElement.className = "smooth";
+  };
+
   testContainer.style.display = "none";
 </script>