Non-composited scrollbars don't fade out when mouse is over.

BUG=669668
TEST=WebFrameTest.TestNonCompositedOverlayScrollbarsFade

Review-Url: https://codereview.chromium.org/2569053002
Cr-Commit-Position: refs/heads/master@{#438178}

third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp

@@ -67,7 +67,8 @@ ScrollableArea::ScrollableArea()
       m_verticalScrollbarNeedsPaintInvalidation(false),
       m_scrollCornerNeedsPaintInvalidation(false),
       m_scrollbarsHidden(false),
-      m_scrollbarCaptured(false) {}
+      m_scrollbarCaptured(false),
+      m_mouseOverScrollbar(false) {}
 
 ScrollableArea::~ScrollableArea() {}
 
@@ -323,13 +324,20 @@ void ScrollableArea::mouseMovedInContentArea() const {
 }
 
 void ScrollableArea::mouseEnteredScrollbar(Scrollbar& scrollbar) {
+  m_mouseOverScrollbar = true;
   scrollAnimator().mouseEnteredScrollbar(scrollbar);
-  // Restart the fade out timer.
   showOverlayScrollbars();
+  if (m_fadeOverlayScrollbarsTimer)
+    m_fadeOverlayScrollbarsTimer->stop();
 }
 
 void ScrollableArea::mouseExitedScrollbar(Scrollbar& scrollbar) {
+  m_mouseOverScrollbar = false;
   scrollAnimator().mouseExitedScrollbar(scrollbar);
+  if (!m_scrollbarsHidden) {
+    // This will kick off the fade out timer.
+    showOverlayScrollbars();
+  }
 }
 
 void ScrollableArea::mouseCapturedScrollbar() {
@@ -574,7 +582,7 @@ void ScrollableArea::showOverlayScrollbars() {
         this, &ScrollableArea::fadeOverlayScrollbarsTimerFired));
   }
 
-  if (!m_scrollbarCaptured) {
+  if (!m_scrollbarCaptured && !m_mouseOverScrollbar) {
     m_fadeOverlayScrollbarsTimer->startOneShot(timeUntilDisable,
                                                BLINK_FROM_HERE);
   }

third_party/WebKit/Source/platform/scroll/ScrollableArea.h

@@ -430,6 +430,7 @@ class PLATFORM_EXPORT ScrollableArea : public GarbageCollectedMixin {
   unsigned m_scrollCornerNeedsPaintInvalidation : 1;
   unsigned m_scrollbarsHidden : 1;
   unsigned m_scrollbarCaptured : 1;
+  unsigned m_mouseOverScrollbar : 1;
 
   // There are 6 possible combinations of writing mode and direction. Scroll
   // origin will be non-zero in the x or y axis if there is any reversed

third_party/WebKit/Source/web/tests/WebFrameTest.cpp

@@ -10982,6 +10982,18 @@ TEST_F(WebFrameTest, TestNonCompositedOverlayScrollbarsFade) {
   testing::runDelayedTasks(kMockOverlayFadeOutDelayMs);
   EXPECT_TRUE(scrollableArea->scrollbarsHidden());
 
+  // Non-composited scrollbars don't fade out while mouse is over.
+  EXPECT_TRUE(scrollableArea->verticalScrollbar());
+  scrollableArea->setScrollOffset(ScrollOffset(20, 20), ProgrammaticScroll,
+                                  ScrollBehaviorInstant);
+  EXPECT_FALSE(scrollableArea->scrollbarsHidden());
+  scrollableArea->mouseEnteredScrollbar(*scrollableArea->verticalScrollbar());
+  testing::runDelayedTasks(kMockOverlayFadeOutDelayMs);
+  EXPECT_FALSE(scrollableArea->scrollbarsHidden());
+  scrollableArea->mouseExitedScrollbar(*scrollableArea->verticalScrollbar());
+  testing::runDelayedTasks(kMockOverlayFadeOutDelayMs);
+  EXPECT_TRUE(scrollableArea->scrollbarsHidden());
+
   mockOverlayTheme.setOverlayScrollbarFadeOutDelay(0.0);
 }