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

--HG--
extra : moz-landing-system : lando
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dholbert committed Feb 6, 2019
1 parent e11487a commit 4147f36
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22 changes: 22 additions & 0 deletions layout/generic/nsGfxScrollFrame.cpp
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Expand Up @@ -977,6 +977,28 @@ static void GetScrollableOverflowForPerspective(
}
}

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::AdjustForPerspective(nsRect& aScrollableOverflow) {
// If we have perspective that is being applied to our children, then
// the effective transform on the child depends on the relative position
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2 changes: 2 additions & 0 deletions layout/generic/nsGfxScrollFrame.h
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Expand Up @@ -812,6 +812,8 @@ class nsHTMLScrollFrame : public nsContainerFrame,
return mHelper.ComputeCustomOverflow(aOverflowAreas);
}

nscoord GetLogicalBaseline(mozilla::WritingMode aWritingMode) const override;

bool GetVerticalAlignBaseline(mozilla::WritingMode aWM,
nscoord* aBaseline) const override {
NS_ASSERTION(!aWM.IsOrthogonalTo(GetWritingMode()),
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58 changes: 58 additions & 0 deletions testing/web-platform/tests/css/css-align/baseline-of-scrollable-1a.html
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@@ -0,0 +1,58 @@
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>
CSS Test: baseline of scrollable element should be taken from its
contents. (Except if the scrollable element is an inline-block, which gets
baseline from its margin-box.)
</title>
<link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
<link rel="author" title="Mozilla" href="https://www.mozilla.org">
<link rel="help" href="https://drafts.csswg.org/css-align/#baseline-export">
<link rel="match" href="reference/baseline-of-scrollable-1-ref.html">
<style>
.container {
overflow: hidden;
height: 50px;
width: 100px;
border-style: solid;
border-width: 2px 3px 4px 5px;
padding: 4px 5px 7px 8px;
margin: 1px 2px 3px 4px;
}
.inline-block {
display: inline-block;
}
.inline-flex {
display: inline-flex;
}
.inline-grid {
display: inline-grid;
}
</style>
</head>
<body>
Test passes if the a/b text aligns with the bottom margin-edge of the "block"
rect and baseline-aligns with the "flex" and "grid" text.
<br><br>

<!-- Note: for this first "inline-block" case, the element's baseline is
synthesized from its margin box. For the other cases, the element's
baseline is taken from its contents, i.e. the text inside of it. -->
a
<div class="container inline-block">block</div>
b
<br>

a
<div class="container inline-flex">flex</div>
b
<br>

a
<div class="container inline-grid">grid</div>
b

</body>
</html>
67 changes: 67 additions & 0 deletions testing/web-platform/tests/css/css-align/baseline-of-scrollable-1b.html
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<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>
CSS Test: baseline of scrollable element (for use by a parent inline-block)
should be taken from its contents. (Except if the scrollable element is a
block, which gets baseline from its margin-box.)
</title>
<link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
<link rel="author" title="Mozilla" href="https://www.mozilla.org">
<link rel="help" href="https://drafts.csswg.org/css-align/#baseline-export">
<link rel="match" href="reference/baseline-of-scrollable-1-ref.html">
<style>
.container {
overflow: hidden;
height: 50px;
width: 100px;
border-style: solid;
border-width: 2px 3px 4px 5px;
padding: 4px 5px 7px 8px;
margin: 1px 2px 3px 4px;
}
.inline-block {
display: inline-block;
}
.block {
display: block;
}
.flex {
display: flex;
}
.grid {
display: grid;
}
</style>
</head>
<body>
Test passes if the a/b text aligns with the bottom margin-edge of the "block"
rect and baseline-aligns with the "flex" and "grid" text.
<br><br>

<!-- Note: for this first "inline-block" case, the element's baseline is
synthesized from its margin box. For the other cases, the element's
baseline is taken from its contents, i.e. the text inside of it. -->
a
<div class="inline-block">
<div class="container block">block</div>
</div>
b
<br>

a
<div class="inline-block">
<div class="container flex">flex</div>
</div>
b
<br>

a
<div class="inline-block">
<div class="container grid">grid</div>
</div>
b

</body>
</html>
58 changes: 58 additions & 0 deletions testing/web-platform/tests/css/css-align/reference/baseline-of-scrollable-1-ref.html
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<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>
CSS Reference Case
</title>
<link rel="author" title="Daniel Holbert" href="mailto:dholbert@mozilla.com">
<link rel="author" title="Mozilla" href="https://www.mozilla.org">
<link rel="help" href="https://drafts.csswg.org/css-align/#baseline-export">
<style>
.container {
/* In this reference case, we leave 'overflow' at its initial value. */
height: 50px;
width: 100px;
border-style: solid;
border-width: 2px 3px 4px 5px;
padding: 4px 5px 7px 8px;
margin: 1px 2px 3px 4px;
}
.inline-block {
display: inline-block;
}
.inline-flex {
display: inline-flex;
}
.inline-grid {
display: inline-grid;
}
</style>
</head>
<body>
Test passes if the a/b text aligns with the bottom margin-edge of the "block"
rect and baseline-aligns with the "flex" and "grid" text.
<br><br>

<!-- Note: for this first "inline-block" case, we take the inner text out of
flow, to force the inline-block to synthesize its baseline from its
margin box. (This is how the corresponding piece of the testcase is
supposed to render). -->
a
<div class="container inline-block">
<div style="position: absolute">block</div>
</div>
b
<br>

a
<div class="container inline-flex">flex</div>
b
<br>

a
<div class="container inline-grid">grid</div>
b

</body>
</html>

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