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LibWeb: Handle negative values when collapsing vertical margins

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In the presence of negative margins, we subtract the largest negative
margin from max(0, largest positive margin).
This commit is contained in:
Andreas Kling 2020-06-12 18:09:40 +02:00
parent 21b1f1653d
commit c91981eba8
3 changed files with 32 additions and 1 deletions
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23
Base/home/anon/www/margin-collapse-2.html Normal file
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@ -0,0 +1,23 @@
<style>
#foo {
border: 1px solid red;
margin-bottom: 25px;
width: 100px;
height: 100px;
}
#bar {
border: 1px solid green;
margin-bottom: 25px;
width: 100px;
height: 100px;
}
#baz {
border: 1px solid blue;
width: 100px;
margin-top: -20px;
height: 100px;
}
</style>
<div id=foo>foo</div>
<div id=bar>bar</div>
<div id=baz>baz</div>

1
Base/home/anon/www/welcome.html
View file

@ -28,6 +28,7 @@ span#ua {
<p>Your user agent is: <b><span id="ua"></span></b></p>
<p>Some small test pages:</p>
<ul>
<li><a href="margin-collapse-2.html">margin collapsing 2</a></li>
<li><a href="margin-collapse-1.html">margin collapsing 1</a></li>
<li><a href="position-absolute-from-edges.html">position: absolute, offset from edges</a></li>
<li><a href="iframe.html">iframe</a></li>

9
Libraries/LibWeb/Layout/LayoutBlock.cpp
View file

@ -481,7 +481,14 @@ void LayoutBlock::compute_position()
// Collapse top margin with bottom margin of previous sibling if necessary
float previous_sibling_margin_bottom = previous_sibling_style.margin().bottom.to_px(*relevant_sibling);
float my_margin_top = box_model().margin().top.to_px(*this);
if (previous_sibling_margin_bottom > my_margin_top) {
if (my_margin_top < 0 || previous_sibling_margin_bottom < 0) {
// Negative margins present.
float largest_negative_margin = -min(my_margin_top, previous_sibling_margin_bottom);
float largest_positive_margin = (my_margin_top < 0 && previous_sibling_margin_bottom < 0) ? 0 : max(my_margin_top, previous_sibling_margin_bottom);
float final_margin = largest_positive_margin - largest_negative_margin;
position_y += final_margin - my_margin_top;
} else if (previous_sibling_margin_bottom > my_margin_top) {
// Sibling's margin is larger than mine, adjust so we use sibling's.
position_y += previous_sibling_margin_bottom - my_margin_top;
}