Remove clips where we can show that they are unnecessary.

Source/core/rendering/RenderBlock.cpp

Index: Source/core/rendering/RenderBlock.cpp
diff --git a/Source/core/rendering/RenderBlock.cpp b/Source/core/rendering/RenderBlock.cpp
index 3b6d47544e3482ca220adb33a23e07597a35cbcb..36b2b7c3a0cc27b0411271050946ccb59005221e 100644
--- a/Source/core/rendering/RenderBlock.cpp
+++ b/Source/core/rendering/RenderBlock.cpp
@@ -1714,6 +1714,15 @@ void RenderBlock::layoutBlock(bool relayoutChildren, LayoutUnit pageLogicalHeigh
     setNeedsLayout(false);
 }
 
+void RenderBlock::addVisualOverflow(const LayoutRect& rect, VisualOverflowClipBehavior clipBehavior)
+{
+    if (clipBehavior == VisualOverflowClippedByContentsClip && hasOverflowClip() && m_rareData) {
+        m_rareData->m_contentsVisualOverflowRect.uniteIfNonZero(rect);

[Julien - ping for review, 2013/06/19 19:42:58]

Why isn't this in RenderOverflow as it is its reason to be?

[jbroman, 2013/08/02 14:28:17]

Moved as requested.

+        return;
+    }
+    RenderBox::addVisualOverflow(rect, clipBehavior);
+}
+
 void RenderBlock::addOverflowFromChildren()
 {
     if (!hasColumns()) {
@@ -1726,8 +1735,7 @@ void RenderBlock::addOverflowFromChildren()
         if (columnCount(colInfo)) {
             LayoutRect lastRect = columnRectAt(colInfo, columnCount(colInfo) - 1);
             addLayoutOverflow(lastRect);
-            if (!hasOverflowClip())
-                addVisualOverflow(lastRect);
+            addVisualOverflow(lastRect, VisualOverflowClippedByContentsClip);
         }
     }
 }
@@ -1735,6 +1743,11 @@ void RenderBlock::addOverflowFromChildren()
 void RenderBlock::computeOverflow(LayoutUnit oldClientAfterEdge, bool recomputeFloats)
 {
     m_overflow.clear();
+    if (hasOverflowClip()) {
+        if (!m_rareData)
+            m_rareData = adoptPtr(new RenderBlockRareData(this));
+        m_rareData->m_contentsVisualOverflowRect = LayoutRect();

[Julien - ping for review, 2013/06/19 19:42:58]

You are now forcing all RenderBlocks with an overflow clip to have a rare data
as computeOverflow is called for each layout. And that is regardless of whether
you will have a need for it.

[jbroman, 2013/08/02 14:28:17]

Changed to RenderOverflow. Note that in practice most elements with overflow
clips will have *some*  contents and will get a RenderOverflow to store their
contents visual bounding rect anyhow. This could be possibly avoided for
"overflow: hidden" but not for "overflow: scroll".

+    }
 
     // Add overflow from children.
     addOverflowFromChildren();
@@ -1759,13 +1772,13 @@ void RenderBlock::computeOverflow(LayoutUnit oldClientAfterEdge, bool recomputeF
         if (hasRenderOverflow())
             m_overflow->setLayoutClientAfterEdge(oldClientAfterEdge);
     }
-        
+
     // Allow our overflow to catch cases where the caret in an empty editable element with negative text indent needs to get painted.
     LayoutUnit textIndent = textIndentOffset();
     if (textIndent < 0) {
         LayoutRect clientRect(clientBoxRect());
-        LayoutRect rectToApply = LayoutRect(clientRect.x() + min<LayoutUnit>(0, textIndent), clientRect.y(), clientRect.width() - min<LayoutUnit>(0, textIndent), clientRect.height());

[Julien - ping for review, 2013/06/19 19:42:58]

Removing the min seems wrong: textIndent is negative in that branch.

[jbroman, 2013/08/02 14:28:17]

min returns the lesser of its arguments. If textIndent < 0, then the lesser of 0
and textIndent is necessarily 0, no?

That said, removing it from this CL to make the diff nicer.

[jbroman, 2013/08/02 15:02:20]

Err, I meant "is necessarily textIndent". The change was correct; my
sentence-forming abilities are questionable.

[Julien - ping for review, 2013/08/07 00:17:08]

Yeah, I misread the condition and your code was right. My bad!

-        addVisualOverflow(rectToApply);
+        LayoutRect rectToApply = LayoutRect(clientRect.x() + textIndent, clientRect.y(), clientRect.width() - textIndent, clientRect.height());
+        addVisualOverflow(rectToApply, VisualOverflowClippedByContentsClip);
     }
 
     // Add visual overflow from box-shadow and border-image-outset.
@@ -1810,14 +1823,10 @@ void RenderBlock::addOverflowFromPositionedObjects()
     TrackedRendererListHashSet::iterator end = positionedDescendants->end();
     for (TrackedRendererListHashSet::iterator it = positionedDescendants->begin(); it != end; ++it) {
         positionedObject = *it;
-        
-        // Fixed positioned elements don't contribute to layout overflow, since they don't scroll with the content.
-        if (positionedObject->style()->position() != FixedPosition) {

[Julien - ping for review, 2013/06/19 19:42:58]

I am not sure I see the point of this change: you are basically still ignoring
fixed positioned objects in addOverflowFromChild as you explicitly ignore them
for layout overflow and they have a self-painting layer. However to arrive at
the same result, you do a lot more computations.

[jbroman, 2013/08/02 14:28:17]

Good point. Reverted.

-            LayoutUnit x = positionedObject->x();
-            if (style()->shouldPlaceBlockDirectionScrollbarOnLogicalLeft())
-                x -= verticalScrollbarWidth();
-            addOverflowFromChild(positionedObject, LayoutSize(x, positionedObject->y()));
-        }
+        LayoutUnit x = positionedObject->x();
+        if (style()->shouldPlaceBlockDirectionScrollbarOnLogicalLeft())
+            x -= verticalScrollbarWidth();
+        addOverflowFromChild(positionedObject, LayoutSize(x, positionedObject->y()));
     }
 }
 
@@ -1828,7 +1837,7 @@ void RenderBlock::addVisualOverflowFromTheme()
 
     IntRect inflatedRect = pixelSnappedBorderBoxRect();
     theme()->adjustRepaintRect(this, inflatedRect);
-    addVisualOverflow(inflatedRect);
+    addVisualOverflow(inflatedRect, VisualOverflowNotClipped);
 }
 
 bool RenderBlock::expandsToEncloseOverhangingFloats() const
@@ -2897,13 +2906,22 @@ void RenderBlock::repaintOverhangingFloats(bool paintAllDescendants)
         }
     }
 }
- 
+
+static bool hasCaret(RenderObject* o)
+{
+    Frame* frame = o->frame();
+    if (frame->selection()->caretRenderer() == o)
+        return frame->selection()->rendererIsEditable() || (frame->settings() && frame->settings()->caretBrowsingEnabled());
+    if (frame->page()->dragCaretController()->caretRenderer() == o)
+        return frame->page()->dragCaretController()->isContentEditable() || (frame->settings() && frame->settings()->caretBrowsingEnabled());
+    return false;
+}

[Julien - ping for review, 2013/06/19 19:42:58]

This should probably be shared with the logic in RenderBlock::paintCaret as they
are very similar.

[jbroman, 2013/08/02 14:28:17]

They diverged and then grew together again. Created RenderBlock::hasCaret to
deduplicate.

+
 void RenderBlock::paint(PaintInfo& paintInfo, const LayoutPoint& paintOffset)
 {
     ANNOTATE_GRAPHICS_CONTEXT(paintInfo, this);
 
     LayoutPoint adjustedPaintOffset = paintOffset + location();
-    
     PaintPhase phase = paintInfo.phase;
 
     // Check if we need to do anything at all.
@@ -2918,7 +2936,14 @@ void RenderBlock::paint(PaintInfo& paintInfo, const LayoutPoint& paintOffset)
             return;
     }
 
-    bool pushedClip = pushContentsClip(paintInfo, adjustedPaintOffset);
+    const LayoutRect* contentsVisualOverflowRect = 0;
+    if (m_rareData && !hasControlClip() && !hasCaret(this) && !(shouldPaintSelectionGaps() && phase == PaintPhaseForeground)) {
+        // Some visual elements clipped by the control clip are not included in m_contentsVisualOverflowRect.
+        // It is also somewhat complicated to determine whether the caret and selection gaps, if present, would be clipped.
+        // FIXME: Determine whether these would be clipped instead of indiscriminately bailing out.
+        contentsVisualOverflowRect = &m_rareData->m_contentsVisualOverflowRect;
+    }
+    bool pushedClip = pushContentsClip(paintInfo, adjustedPaintOffset, contentsVisualOverflowRect);
     paintObject(paintInfo, adjustedPaintOffset);
     if (pushedClip)
         popContentsClip(paintInfo, phase, adjustedPaintOffset);
@@ -3215,7 +3240,7 @@ void RenderBlock::paintObject(PaintInfo& paintInfo, const LayoutPoint& paintOffs
     // 7. paint caret.
     // If the caret's node's render object's containing block is this block, and the paint action is PaintPhaseForeground,
     // then paint the caret.
-    if (paintPhase == PaintPhaseForeground) {        
+    if (paintPhase == PaintPhaseForeground) {
         paintCaret(paintInfo, paintOffset, CursorCaret);
         paintCaret(paintInfo, paintOffset, DragCaret);
     }