Print with the correct physical dimensions when specified

third_party/WebKit/Source/core/page/PrintContext.cpp

Index: third_party/WebKit/Source/core/page/PrintContext.cpp
diff --git a/third_party/WebKit/Source/core/page/PrintContext.cpp b/third_party/WebKit/Source/core/page/PrintContext.cpp
index e0713d58ffd2bd9d29ce883a064900b16b132fc5..1f12992c82c159cbc0ffa38377657ce5af069aa5 100644
--- a/third_party/WebKit/Source/core/page/PrintContext.cpp
+++ b/third_party/WebKit/Source/core/page/PrintContext.cpp
@@ -27,17 +27,17 @@
 
 namespace blink {
 
-// By imaging to a width a little wider than the available pixels,
-// thin pages will be scaled down a little, matching the way they
-// print in IE and Camino. This lets them use fewer sheets than they
-// would otherwise, which is presumably why other browsers do this.
+// By shrinking to a width of 75% (1.333f) we will render the correct physical
+// dimensions in paged media (i.e. cm, pt,). The shrinkage used
+// to be 80% (1.25f) to match other browsers - they have since moved on.
 // Wide pages will be scaled down more than this.
-const float printingMinimumShrinkFactor = 1.25f;
+const float printingMinimumShrinkFactor = 1.333f;
 
 // This number determines how small we are willing to reduce the page content
 // in order to accommodate the widest line. If the page would have to be
 // reduced smaller to make the widest line fit, we just clip instead (this
-// behavior matches MacIE and Mozilla, at least)
+// behavior matches MacIE and Mozilla, at least).
+// TODO(rhogan): Decide if this quirk is still required.
 const float printingMaximumShrinkFactor = 2;
 
 PrintContext::PrintContext(LocalFrame* frame)
@@ -99,7 +99,10 @@ void PrintContext::computePageRectsWithPageSizeInternal(const FloatSize& pageSiz
     IntRect docRect = view->documentRect();
 
     int pageWidth = pageSizeInPixels.width();
-    int pageHeight = pageSizeInPixels.height();
+    // We scaled with floating point arithmetic and need to ensure results like 13329.99
+    // are treated as 13330 so that we don't mistakenly assign an extra page for the
+    // stray pixel.
+    int pageHeight = pageSizeInPixels.height() + LayoutUnit::epsilon();

[rhogan, 2016/03/29 19:17:23]

eae: I added this change since you lgtm'd it. Can you TAL?

[eae, 2016/03/29 19:28:47]

I'd prefer if we snapped it rather than added epsilon but I trust your
judgement.

 
     bool isHorizontal = view->style()->isHorizontalWritingMode();