Bug 969874: Make scrollable frames derive their baseline from their s…

…crolled content (unless their display value is block-inside). r=mats

Before this patch, we made scrollable frames derive their baseline from their
margin-box, because that's what the spec requires for scrollable inline-block
boxes. However, the spec now singles out inline-block as a special case, and
other sorts of scrollable inline-level containers are supposed to derive their
baseline from the scrolled content. So, this patch makes us do that, with an
exception for scrollable inline-block boxes.

For more info about the block-inside special case, see the end of the "block
containers" chunk here: https://drafts.csswg.org/css-align/#baseline-export
(Though that spec text may be a bit too specific, per my spec issue at
w3c/csswg-drafts#3611 -- that's why this patch checks
for block-inside rather than inline-block.)

Differential Revision: https://phabricator.services.mozilla.com/D18481

UltraBlame original commit: 21b74260c8a9fcc8c35ffafc2f98254753fe8d7a

layout/generic/nsGfxScrollFrame.cpp

@@ -7463,6 +7463,218 @@ GetScrollableOverflowRectRelativeToParent
 }
 }
 }
+nscoord
+nsHTMLScrollFrame
+:
+:
+GetLogicalBaseline
+(
+WritingMode
+aWritingMode
+)
+const
+{
+/
+/
+This
+function
+implements
+some
+of
+the
+spec
+text
+here
+:
+/
+/
+https
+:
+/
+/
+drafts
+.
+csswg
+.
+org
+/
+css
+-
+align
+/
+#
+baseline
+-
+export
+/
+/
+/
+/
+Specifically
+:
+if
+our
+scrolled
+frame
+is
+a
+block
+we
+just
+use
+the
+inherited
+/
+/
+GetLogicalBaseline
+(
+)
+impl
+which
+synthesizes
+a
+baseline
+from
+the
+/
+/
+margin
+-
+box
+.
+Otherwise
+we
+defer
+to
+our
+scrolled
+frame
+considering
+it
+/
+/
+to
+be
+scrolled
+to
+its
+initial
+scroll
+position
+.
+if
+(
+mHelper
+.
+mScrolledFrame
+-
+>
+IsBlockFrame
+(
+)
+)
+{
+return
+nsContainerFrame
+:
+:
+GetLogicalBaseline
+(
+aWritingMode
+)
+;
+}
+/
+/
+OK
+here
+'
+s
+where
+we
+defer
+to
+our
+scrolled
+frame
+.
+We
+have
+to
+add
+our
+/
+/
+border
+BStart
+thickness
+to
+whatever
+it
+returns
+to
+produce
+an
+offset
+in
+/
+/
+our
+frame
+-
+rect
+'
+s
+coordinate
+system
+.
+(
+We
+don
+'
+t
+have
+to
+add
+padding
+/
+/
+because
+the
+scrolled
+frame
+handles
+our
+padding
+.
+)
+LogicalMargin
+border
+=
+GetLogicalUsedBorder
+(
+aWritingMode
+)
+;
+return
+border
+.
+BStart
+(
+aWritingMode
+)
++
+mHelper
+.
+mScrolledFrame
+-
+>
+GetLogicalBaseline
+(
+aWritingMode
+)
+;
+}
 void
 nsHTMLScrollFrame
 :

layout/generic/nsGfxScrollFrame.h

@@ -5238,6 +5238,18 @@ aOverflowAreas
 )
 ;
 }
+nscoord
+GetLogicalBaseline
+(
+mozilla
+:
+:
+WritingMode
+aWritingMode
+)
+const
+override
+;
 bool
 GetVerticalAlignBaseline
 (