Layout 2020: Properly handle negative block margins in floats

If a float has negative block margins, it should be pushed upward, but
shouldn't affect the positioning of any floats that came before it. It
should lower the ceiling though when it still has some non-negative
block contribution. In order to implement this behavior, we should only
place the float considering its non-negative block length contribution. If
the float is pushed up completely past it's "natural" position, it
should be placed like a float with zero block size.

4 files changed, 49 insertions, 4 deletions

diff --git a/components/layout_2020/flow/float.rs b/components/layout_2020/flow/float.rs
index 784c5d62722..0ac6529278a 100644
--- a/components/layout_2020/flow/float.rs
+++ b/components/layout_2020/flow/float.rs
@@ -25,7 +25,7 @@ use std::{f32, mem};
 use style::computed_values::clear::T as ClearProperty;
 use style::computed_values::float::T as FloatProperty;
 use style::properties::ComputedValues;
-use style::values::computed::Length;
+use style::values::computed::{CSSPixelLength, Length};
 use style::values::specified::text::TextDecorationLine;
 
 /// A floating box.
@@ -191,14 +191,22 @@ impl FloatContext {
     /// Places a new float and adds it to the list. Returns the start corner of its margin box.
     pub fn add_float(&mut self, new_float: &PlacementInfo) -> Vec2<Length> {
         // Place the float.
-        let new_float_origin = self.place_object(new_float);
+        let new_float_origin = self.place_object(&new_float);
         let new_float_extent = match new_float.side {
             FloatSide::Left => new_float_origin.inline + new_float.size.inline,
             FloatSide::Right => new_float_origin.inline,
         };
+
         let new_float_rect = Rect {
             start_corner: new_float_origin,
-            size: new_float.size.clone(),
+            // If this float has a negative margin, we should only consider its non-negative
+            // block size contribution when determing where to place it. When the margin is
+            // so negative that it's placed completely above the current float ceiling, then
+            // we should position it as if it had zero block size.
+            size: Vec2 {
+                inline: new_float.size.inline.max(CSSPixelLength::zero()),
+                block: new_float.size.block.max(CSSPixelLength::zero()),
+            },
         };
 
         // Update clear.
@@ -759,7 +767,6 @@ impl FloatBox {
                             .make_fragments(&replaced.style, content_size.clone());
                     },
                 };
-
                 let margin_box_start_corner = float_context.add_float(&PlacementInfo {
                     size: &content_size + &pbm_sums.sum(),
                     side: FloatSide::from_style(&style).expect("Float box wasn't floated!"),
diff --git a/tests/wpt/metadata/MANIFEST.json b/tests/wpt/metadata/MANIFEST.json
index 8e22e1ee7e1..efebc16e140 100644
--- a/tests/wpt/metadata/MANIFEST.json
+++ b/tests/wpt/metadata/MANIFEST.json
@@ -60647,6 +60647,19 @@
         {}
        ]
       ],
+      "negative-block-margin-pushing-float-out-of-block-formatting-context.html": [
+       "0ebe54c1904e0c2487ebe15c4d60e9e93672d859",
+       [
+        null,
+        [
+         [
+          "/css/CSS2/floats/negative-block-margin-pushing-float-out-of-block-formatting-context-ref.html",
+          "=="
+         ]
+        ],
+        {}
+       ]
+      ],
       "negative-margin-float-positioning.html": [
        "8b4aef22f71f8695ebbc5b6dc83ad5639945618d",
        [
@@ -359477,6 +359490,10 @@
        "68139cdbe2d7c998b6d948f5ecca0ed7ce367694",
        []
       ],
+      "negative-block-margin-pushing-float-out-of-block-formatting-context-ref.html": [
+       "09a0ed094f8eb82f9fa25c4f09bb3f8a1f79c4c9",
+       []
+      ],
       "overflow-scroll-float-paint-order-ref.html": [
        "0fb53d4ecd3cd77e6588be0b271a2ee47950bd29",
        []
diff --git a/tests/wpt/web-platform-tests/css/CSS2/floats/negative-block-margin-pushing-float-out-of-block-formatting-context-ref.html b/tests/wpt/web-platform-tests/css/CSS2/floats/negative-block-margin-pushing-float-out-of-block-formatting-context-ref.html
new file mode 100644
index 00000000000..09a0ed094f8
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/CSS2/floats/negative-block-margin-pushing-float-out-of-block-formatting-context-ref.html
@@ -0,0 +1,8 @@
+<!DOCTYPE html>
+
+<link rel="author" title="Martin Robinson" href="mailto:mrobinson@igalia.com" />
+<meta name="assert" content="When a floating box has a negative margin negative enough that it pushes the float past the top of the block formatting context, it should render properly."/>
+
+<body>
+<div style="width: 50px; height: 50px; background: green;"></div>
+</body>
diff --git a/tests/wpt/web-platform-tests/css/CSS2/floats/negative-block-margin-pushing-float-out-of-block-formatting-context.html b/tests/wpt/web-platform-tests/css/CSS2/floats/negative-block-margin-pushing-float-out-of-block-formatting-context.html
new file mode 100644
index 00000000000..0ebe54c1904
--- /dev/null
+++ b/tests/wpt/web-platform-tests/css/CSS2/floats/negative-block-margin-pushing-float-out-of-block-formatting-context.html
@@ -0,0 +1,13 @@
+<!DOCTYPE html>
+
+<link rel="author" title="Martin Robinson" href="mailto:mrobinson@igalia.com" />
+<link rel="help" href="https://drafts.csswg.org/css2/#float-position"/>
+<meta name="assert" content="When a floating box has a negative margin negative enough that it pushes the float past the top of the block formatting context, it should render properly."/>
+<link rel="match" href="negative-block-margin-pushing-float-out-of-block-formatting-context-ref.html"/>
+
+<body>
+<div style="height: 100px;"></div>
+<div style="position: absolute;">
+    <div style="float: left; width: 50px; height: 50px; margin-top: -100px; background: green;"></div>
+</div>
+</body>