aw: Add rounding to BVR::SetTotalRootLayerScrollOffset() calculation.

The intermediate multiplication result was too large to store in float
precisely, therefore there was a rounding error. To mitigate this, we
apply a rounding for the integer conversion.

We only need this for scale up since the granularity is finer.

Bug: 881458
Change-Id: I9c07fdfb71fcbc59f36f5094bf813053cac8d8e8
Reviewed-on: https://chromium-review.googlesource.com/1214686
Commit-Queue: Shimi Zhang <ctzsm@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590147}

android_webview/browser/browser_view_renderer.cc

@@ -631,13 +631,15 @@ void BrowserViewRenderer::SetTotalRootLayerScrollOffset(
   // For an explanation as to why this is done this way see the comment in
   // BrowserViewRenderer::ScrollTo.
   if (max_scroll_offset_unscaled_.x()) {
-    scroll_offset.set_x((scroll_offset_unscaled.x() * max_offset.x()) /
-                        max_scroll_offset_unscaled_.x());
+    scroll_offset.set_x(
+        gfx::ToRoundedInt((scroll_offset_unscaled.x() * max_offset.x()) /
+                          max_scroll_offset_unscaled_.x()));
   }
 
   if (max_scroll_offset_unscaled_.y()) {
-    scroll_offset.set_y((scroll_offset_unscaled.y() * max_offset.y()) /
-                        max_scroll_offset_unscaled_.y());
+    scroll_offset.set_y(
+        gfx::ToRoundedInt((scroll_offset_unscaled.y() * max_offset.y()) /
+                          max_scroll_offset_unscaled_.y()));
   }
 
   DCHECK_LE(0, scroll_offset.x());

android_webview/browser/browser_view_renderer_unittest.cc

@@ -547,4 +547,51 @@ class RenderThreadManagerSwitchTest : public ResourceRenderingTest {
 
 RENDERING_TEST_F(RenderThreadManagerSwitchTest);
 
+// Test for https://crbug.com/881458, this test is to make sure we will reach
+// the maximal scroll offset.
+class DidReachMaximalScrollOffsetTest : public RenderingTest {
+ public:
+  void StartTest() override {
+    browser_view_renderer_->SetDipScale(kDipScale);
+    // |UpdateRootLayerState()| will call |SetTotalRootLayerScrollOffset()|.
+    browser_view_renderer_->UpdateRootLayerState(
+        ActiveCompositor(), kTotalScrollOffset, kTotalMaxScrollOffset,
+        kScrollableSize, kPageScaleFactor, kMinPageScaleFactor,
+        kMaxPageScaleFactor);
+  }
+
+  void ScrollContainerViewTo(const gfx::Vector2d& new_value) override {
+    EXPECT_EQ(kExpectedScrollOffset.ToString(), new_value.ToString());
+    EndTest();
+  }
+
+ private:
+  static constexpr float kDipScale = 2.625f;
+  static const gfx::Vector2dF kTotalScrollOffset;
+  static const gfx::Vector2dF kTotalMaxScrollOffset;
+  static const gfx::SizeF kScrollableSize;
+  static constexpr float kPageScaleFactor = 1.f;
+  // These two are not used in this test.
+  static constexpr float kMinPageScaleFactor = 1.f;
+  static constexpr float kMaxPageScaleFactor = 5.f;
+
+  static const gfx::Vector2d kExpectedScrollOffset;
+};
+
+// The current scroll offset in logical pixel, which is at the end.
+const gfx::Vector2dF DidReachMaximalScrollOffsetTest::kTotalScrollOffset =
+    gfx::Vector2dF(0.f, 6132.f);
+// The maximum possible scroll offset in logical pixel.
+const gfx::Vector2dF DidReachMaximalScrollOffsetTest::kTotalMaxScrollOffset =
+    gfx::Vector2dF(0.f, 6132.f);
+// This is what passed to CTS test, not used for this test.
+const gfx::SizeF DidReachMaximalScrollOffsetTest::kScrollableSize =
+    gfx::SizeF(412.f, 6712.f);
+// In max_scroll_offset() we are using ceiling rounding for scaled scroll
+// offset. Therefore ceiling(2.625 * 6132 = 16096.5) = 16097.
+const gfx::Vector2d DidReachMaximalScrollOffsetTest::kExpectedScrollOffset =
+    gfx::Vector2d(0, 16097);
+
+RENDERING_TEST_F(DidReachMaximalScrollOffsetTest);
+
 }  // namespace android_webview