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

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

Index: third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp b/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
index 6d5b7a1d6413667c0cf6b8b610cbe2324d75243e..1ddc3e7c2e6c48332b8b2ac4a6b84344ce7f13f3 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
@@ -60,9 +60,12 @@ struct SameSizeAsScrollableArea {
 
 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()
@@ -566,7 +569,7 @@ DoublePoint ScrollableArea::clampScrollPosition(const DoublePoint& scrollPositio
 
 int ScrollableArea::lineStep(ScrollbarOrientation) const
 {
-    return pixelsPerLineStep();
+    return pixelsPerLineStep(hostWindow());
 }
 
 int ScrollableArea::pageStep(ScrollbarOrientation orientation) const
@@ -586,6 +589,8 @@ int ScrollableArea::documentStep(ScrollbarOrientation orientation) const
 
 float ScrollableArea::pixelStep(ScrollbarOrientation) const
 {
+    if (HostWindow* window = hostWindow())
+        return window->windowToViewport(FloatRect(1, 0, 0, 0)).x();
     return 1;
 }