Improve mobile device rotation behavior.

Source/WebKit/chromium/src/WebViewImpl.cpp

Index: Source/WebKit/chromium/src/WebViewImpl.cpp
diff --git a/Source/WebKit/chromium/src/WebViewImpl.cpp b/Source/WebKit/chromium/src/WebViewImpl.cpp
index 0a7b1f89494d0fbbcfd72c97ca7d33e6ddc9ec00..73a530167506f12991c4f63759c2e487a352f32d 100644
--- a/Source/WebKit/chromium/src/WebViewImpl.cpp
+++ b/Source/WebKit/chromium/src/WebViewImpl.cpp
@@ -73,7 +73,6 @@
 #include "HTMLMediaElement.h"
 #include "HTMLNames.h"
 #include "HTMLTextAreaElement.h"
-#include "HitTestResult.h"
 #include "Image.h"
 #include "ImageBuffer.h"
 #include "InspectorController.h"
@@ -119,6 +118,7 @@
 #include "TouchDisambiguation.h"
 #include "TraceEvent.h"
 #include "ValidationMessageClientImpl.h"
+#include "ViewportAnchor.h"
 #include "WebAccessibilityObject.h"
 #include "WebActiveWheelFlingParameters.h"
 #include "WebAutofillClient.h"
@@ -197,6 +197,10 @@ static const float doubleTapZoomContentMinimumMargin = 2;
 static const double doubleTapZoomAnimationDurationInSeconds = 0.25;
 static const float doubleTapZoomAlreadyLegibleRatio = 1.2f;
 
+// Constants for viewport anchoring on resize.
+static const float viewportAnchorXCoord = 0.5f;
+static const float viewportAnchorYCoord = 0;
+
 // Constants for zooming in on a focused text field.
 static const double scrollAndScaleAnimationDurationInSeconds = 0.2;
 static const int minReadableCaretHeight = 18;
@@ -1654,12 +1658,18 @@ void WebViewImpl::resize(const WebSize& newSize)
 
     WebSize oldSize = m_size;
     float oldPageScaleFactor = pageScaleFactor();
-    IntSize oldScrollOffset = view->scrollOffset();
+    float oldMinimumPageScaleFactor = m_minimumPageScaleFactor;
     int oldFixedLayoutWidth = fixedLayoutSize().width;
+    int oldContentsWidth = contentsSize().width();
 
     m_size = newSize;
 
 #if ENABLE(VIEWPORT)
+    ViewportAnchor viewportAnchor(mainFrameImpl()->frame()->eventHandler());
+    if (settings()->viewportEnabled())
+        viewportAnchor.setAnchor(view->visibleContentRect(),
+                                 FloatSize(viewportAnchorXCoord, viewportAnchorYCoord));
+
     ViewportArguments viewportArguments = mainFrameImpl()->frame()->document()->viewportArguments();
     m_page->chrome()->client()->dispatchViewportPropertiesDidChange(viewportArguments);
 #endif
@@ -1679,21 +1689,29 @@ void WebViewImpl::resize(const WebSize& newSize)
         if (view->needsLayout())
             view->layout();
 
-        // When the device rotates:
-        // - If the page width is unchanged, then zoom by new width/old width
-        //   such as to keep the same content horizontally onscreen.
-        // - If the page width stretches proportionally to the change in
-        //   screen width, then don't zoom at all (assuming the content has
-        //   scaled uniformly, then the same content will be horizontally
-        //   onscreen).
-        //   - If the page width partially stretches, then zoom partially to
-        //   make up the difference.
-        // In all cases try to keep the same content at the top of the screen.
+        // The minimum scale limit may change as a result of layout content
+        // changes; in this case, scale relative to the change in content width.
+        // Otherwise scale relative to the change in layout width.
+        // Don't do any scaling if the old width was zero (i.e., first resize).
+        float scalingWidthRatio = 1;
+        if (oldSize.width && oldContentsWidth && m_minimumPageScaleFactor != oldMinimumPageScaleFactor)

[aelias_OOO_until_Jul13, 2013/04/15 18:25:39]

Could you delete the "&& m_minimumPageScaleFactor != oldMinimumPageScaleFactor"?
 It seems that in this case, oldContentsWidth == contentSize.width() so you can
safely do the division and get 1.

[jdduke (slow), 2013/04/16 15:40:26]

Done.

+            scalingWidthRatio = contentsSize().width() / (float) oldContentsWidth;
+        else if (oldFixedLayoutWidth)
+            scalingWidthRatio = fixedLayoutSize().width / (float) oldFixedLayoutWidth;

[aelias_OOO_until_Jul13, 2013/04/15 18:25:39]

Could you try deleting this else block and see if anything breaks?  Contents
width should be set to the greater of fixed layout width or greatest node width.
 So the fixedLayoutSize information is largely contained within contentsWidth
anyway and I'm not sure this covers any other relevant case.

[jdduke (slow), 2013/04/16 15:40:26]

This initially caused some problems with fixed width pages, which is why I left
it in.  However, adding an additional check on "oldSize.width" when using the
contentsSize.width() ratio fixes the problem. This is much cleaner.

+
         float viewportWidthRatio = !oldSize.width ? 1 : newSize.width / (float) oldSize.width;
-        float fixedLayoutWidthRatio = !oldFixedLayoutWidth ? 1 : fixedLayoutSize().width / (float) oldFixedLayoutWidth;
-        float scaleMultiplier = viewportWidthRatio / fixedLayoutWidthRatio;
-        if (scaleMultiplier != 1)
-            setPageScaleFactor(oldPageScaleFactor * scaleMultiplier, WebPoint(oldScrollOffset.width(), oldScrollOffset.height()));
+        float scaleMultiplier = viewportWidthRatio / scalingWidthRatio;
+
+        IntSize viewportSize = view->visibleContentRect().size();
+        if (scaleMultiplier != 1) {
+            float newPageScaleFactor = oldPageScaleFactor * scaleMultiplier;
+            viewportSize.scale(pageScaleFactor() / newPageScaleFactor);
+            IntPoint scrollOffsetAtNewScale = viewportAnchor.computeOrigin(viewportSize);
+            setPageScaleFactor(newPageScaleFactor, scrollOffsetAtNewScale);
+        } else {
+            IntPoint scrollOffsetAtNewScale = clampOffsetAtScale(viewportAnchor.computeOrigin(viewportSize), pageScaleFactor());
+            updateMainFrameScrollPosition(scrollOffsetAtNewScale, false);
+        }
     }
 #endif