Disable scroll anchoring when in printing mode

third_party/WebKit/Source/core/frame/FrameView.h

Index: third_party/WebKit/Source/core/frame/FrameView.h
diff --git a/third_party/WebKit/Source/core/frame/FrameView.h b/third_party/WebKit/Source/core/frame/FrameView.h
index d8f183533b304cbc72c827789d3e7ae11255013c..2d8fb9c4259920ba0e16984f38f7091da9042be7 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.h
+++ b/third_party/WebKit/Source/core/frame/FrameView.h
@@ -727,6 +727,13 @@ class CORE_EXPORT FrameView final
   // PaintInvalidator.
   void invalidatePaintIfNeeded(const PaintInvalidationState&);
 
+  bool disableScrollAnchoringForPrinting() {
+    return m_disableScrollAnchoringForPrinting;
+  }
+  void setDisableScrollAnchoringForPrinting(bool value) {
+    m_disableScrollAnchoringForPrinting = value;
+  }
+
   // Only for SPv2.
   std::unique_ptr<JSONObject> compositedLayersAsJSON(LayerTreeFlags);
 
@@ -1058,6 +1065,10 @@ class CORE_EXPORT FrameView final
   DocumentLifecycle::LifecycleState m_currentUpdateLifecyclePhasesTargetState;
 
   ScrollAnchor m_scrollAnchor;
+  // This is set when we're in printing mode. We can't reuse the m_printing bit
+  // from blink::Document because it's reset before the set of layouts caused

[skobes, 2016/10/12 22:40:31]

This seems wrong.  Isn't Document::m_printing cleared by PrintContext::end() at
the same time you're clearing this new bit?

[ymalik, 2016/10/13 13:48:04]

Not quite. Document::m_printing is cleared from LocalFrame::setPrinting, before
the layout that trigger unnecessary adjustments. 

See
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/frame/Loc...

Another potential fix is clearing the bit at the end of the
LocalFrame::setPrinting function, but I am afraid it has a lot of users and its
not clear to me whether the order matters.

[skobes, 2016/10/13 16:24:00]

Ah, I suspect LocalFrame::setPrinting does a layout after clearing m_printing in
order to reset the layout tree to its original state, and the anchor node moves
during this layout.

I'd rather turn Document::m_printing into an enum {NotPrinting, Printing,
ExitingPrinting} than add a new bit on FrameView for this.  But I also see we
have Document::wasPrinting() added to suppress CSS transitions in
http://crbug.com/412281.  Is it feasible to use that?

[ymalik, 2016/10/14 16:30:56]

Yes an enum is cleaner in this case. It also allows us to remove
Document::wasPrinting()

Document::wasPrinting is used to disable css transitions in PrintMode. Basically
the call stack is as follows:
PrintContext::Begin
Update CSS Animation (m_printing = 1, m_wasPrinting = 0) - Don't update
transition
Document::UpdateState (m_wasPrinting = 1)
layout for printing mode
...
PrintContext::End
Update CSS Animation (m_printing = 0, m_wasPrinting = 1) - Don't update
transition
Document::UpdateState (m_wasPrinting = 0)
layout to restore original state

We couldn't use m_wasPrinting in the case of scroll anchoring because it was
reset before layout. Using this enum, we can cover both the transition case and
scroll anchoring case. I verified locally that this works.

+  // by printing.
+  bool m_disableScrollAnchoringForPrinting;
 
   bool m_needsScrollbarsUpdate;
   bool m_suppressAdjustViewSize;