ScrollableArea::pixelsPerLine should return viewport pixels in use-zoom-for-dsf mode

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

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  * Copyright (C) 2008, 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 42 matching lines @@
 #if ENABLE(ASSERT) && ENABLE(OILPAN)
     VerifyEagerFinalization verifyEager;
 #endif
     OwnPtrWillBeMember<void*> pointer[2];
     unsigned bitfields : 16;
     IntPoint origin;
 };
 
 static_assert(sizeof(ScrollableArea) == sizeof(SameSizeAsScrollableArea), "ScrollableArea should stay small");
 
-int ScrollableArea::pixelsPerLineStep()
+int ScrollableArea::pixelsPerLineStep(HostWindow* host)
 {
-    return kPixelsPerLineStep;
+    if (!host)
+        return kPixelsPerLineStep;
+    FloatRect tmpRectInViewport = host->windowToViewport(FloatRect(1, 0, 0, 0));

[skobes, 2016/02/09 23:42:32]

Can we introduce a clearer API that doesn't use rects for one-dimensional unit
conversions?  Something like HostWindow::convertDIPToViewportPx(float) -> float?

[oshima, 2016/02/16 06:09:53]

I was asked to either made this symmetric to viewportToWindow in
https://codereview.chromium.org/1603253003/ (or return scale factor, which I
didn't want because it's not correct in traditional mode). 

I changed the method to return scale var in HostWindow only because all user of
HostWindow are scalar. Please let me know what you think.

+    return kPixelsPerLineStep * tmpRectInViewport.x();
 }
 
 float ScrollableArea::minFractionToStepWhenPaging()
 {
     return kMinFractionToStepWhenPaging;
 }
 
 int ScrollableArea::maxOverlapBetweenPages()
 {
     static int maxOverlapBetweenPages = ScrollbarTheme::theme().maxOverlapBetweenPages();
@@ skipping 483 matching lines @@
     return scrollPosition.shrunkTo(maximumScrollPosition()).expandedTo(minimumScrollPosition());
 }
 
 DoublePoint ScrollableArea::clampScrollPosition(const DoublePoint& scrollPosition) const
 {
     return scrollPosition.shrunkTo(maximumScrollPositionDouble()).expandedTo(minimumScrollPositionDouble());
 }
 
 int ScrollableArea::lineStep(ScrollbarOrientation) const
 {
-    return pixelsPerLineStep();
+    return pixelsPerLineStep(hostWindow());
 }
 
 int ScrollableArea::pageStep(ScrollbarOrientation orientation) const
 {
     IntRect visibleRect = visibleContentRect(IncludeScrollbars);
     int length = (orientation == HorizontalScrollbar) ? visibleRect.width() : visibleRect.height();
     int minPageStep = static_cast<float>(length) * minFractionToStepWhenPaging();
     int pageStep = std::max(minPageStep, length - maxOverlapBetweenPages());
 
     return std::max(pageStep, 1);
 }
 
 int ScrollableArea::documentStep(ScrollbarOrientation orientation) const
 {
     return scrollSize(orientation);
 }
 
 float ScrollableArea::pixelStep(ScrollbarOrientation) const
 {
+    if (HostWindow* window = hostWindow())
+        return window->windowToViewport(FloatRect(1, 0, 0, 0)).x();
     return 1;
 }
 
 IntSize ScrollableArea::excludeScrollbars(const IntSize& size) const
 {
     int verticalScrollbarWidth = 0;
     int horizontalScrollbarHeight = 0;
 
     if (Scrollbar* verticalBar = verticalScrollbar())
         verticalScrollbarWidth = !verticalBar->isOverlayScrollbar() ? verticalBar->width() : 0;
     if (Scrollbar* horizontalBar = horizontalScrollbar())
         horizontalScrollbarHeight = !horizontalBar->isOverlayScrollbar() ? horizontalBar->height() : 0;
 
     return IntSize(std::max(0, size.width() - verticalScrollbarWidth),
         std::max(0, size.height() - horizontalScrollbarHeight));
 }
 
 DEFINE_TRACE(ScrollableArea)
 {
     visitor->trace(m_scrollAnimator);
     visitor->trace(m_programmaticScrollAnimator);
 }
 
 } // namespace blink