layout: Only take nonnegative margins into account when estimating

inline placement of floats.

Otherwise, the heuristics can pass even when there are no floats,
causing block formatting contexts to be speculated to be flowing around
floats that don't exist!

Closes #10237.

components/layout/floats.rs

@@ -459,8 +459,8 @@ impl SpeculatedFloatPlacement {
         }
     }
 
-    /// Given the speculated inline size of the floats in for this flow, computes the speculated
-    /// inline size of the floats out for this flow.
+    /// Given the speculated inline size of the floats out for this flow's last child, computes the
+    /// speculated inline size of the floats out for this flow.
     pub fn compute_floats_out(&mut self, flow: &mut Flow) {
         if flow.is_block_like() {
             let block_flow = flow.as_block();
@@ -470,10 +470,10 @@ impl SpeculatedFloatPlacement {
                 if self.left > Au(0) || self.right > Au(0) {
                     let speculated_inline_content_edge_offsets =
                         block_flow.fragment.guess_inline_content_edge_offsets();
-                    if self.left > Au(0) {
+                    if self.left > Au(0) && speculated_inline_content_edge_offsets.start > Au(0) {
                         self.left = self.left + speculated_inline_content_edge_offsets.start
                     }
-                    if self.right > Au(0) {
+                    if self.right > Au(0) && speculated_inline_content_edge_offsets.end > Au(0) {
                         self.right = self.right + speculated_inline_content_edge_offsets.end
                     }
                 }
@@ -507,16 +507,20 @@ impl SpeculatedFloatPlacement {
         let speculated_inline_content_edge_offsets =
             parent_block_flow.fragment.guess_inline_content_edge_offsets();
 
-        if placement.left > speculated_inline_content_edge_offsets.start {
-            placement.left = placement.left - speculated_inline_content_edge_offsets.start
-        } else {
-            placement.left = Au(0)
-        };
-        if placement.right > speculated_inline_content_edge_offsets.end {
-            placement.right = placement.right - speculated_inline_content_edge_offsets.end
-        } else {
-            placement.right = Au(0)
-        };
+        if speculated_inline_content_edge_offsets.start > Au(0) {
+            placement.left = if placement.left > speculated_inline_content_edge_offsets.start {
+                placement.left - speculated_inline_content_edge_offsets.start
+            } else {
+                Au(0)
+            }
+        }
+        if speculated_inline_content_edge_offsets.end > Au(0) {
+            placement.right = if placement.right > speculated_inline_content_edge_offsets.end {
+                placement.right - speculated_inline_content_edge_offsets.end
+            } else {
+                Au(0)
+            }
+        }
 
         placement
     }

tests/wpt/mozilla/meta/MANIFEST.json

@@ -1484,6 +1484,18 @@
             "url": "/_mozilla/css/float_right_intrinsic_width_a.html"
           }
         ],
+        "css/float_speculation_negative_inline_margins_a.html": [
+          {
+            "path": "css/float_speculation_negative_inline_margins_a.html",
+            "references": [
+              [
+                "/_mozilla/css/float_speculation_negative_inline_margins_ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/css/float_speculation_negative_inline_margins_a.html"
+          }
+        ],
         "css/float_table_a.html": [
           {
             "path": "css/float_table_a.html",
@@ -7822,6 +7834,18 @@
           "url": "/_mozilla/css/float_right_intrinsic_width_a.html"
         }
       ],
+      "css/float_speculation_negative_inline_margins_a.html": [
+        {
+          "path": "css/float_speculation_negative_inline_margins_a.html",
+          "references": [
+            [
+              "/_mozilla/css/float_speculation_negative_inline_margins_ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/css/float_speculation_negative_inline_margins_a.html"
+        }
+      ],
       "css/float_table_a.html": [
         {
           "path": "css/float_table_a.html",

tests/wpt/mozilla/tests/css/float_speculation_negative_inline_margins_a.html

@@ -0,0 +1,30 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title></title>
+<link rel="match" href="float_speculation_negative_inline_margins_ref.html">
+<style>
+  * {
+    margin: 0;
+    padding: 0;
+    font-size: 0;
+  }
+  .red {
+    left: 50%;
+    position: absolute;
+    top: 40px;
+    width: 480px;
+    margin-left: -240px;
+    background: red;
+  }
+  .green {
+    display: block;
+    width: 410px;
+    height: 34px;
+    line-height: 34px;
+    background: green;
+    border: none
+  }
+</style>
+
+<div class="red"><input class="green"></div>
+

tests/wpt/mozilla/tests/css/float_speculation_negative_inline_margins_ref.html

@@ -0,0 +1,29 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title></title>
+<style>
+  * {
+    margin: 0;
+    padding: 0;
+    font-size: 0;
+  }
+  .red {
+    left: 50%;
+    position: absolute;
+    top: 40px;
+    width: 480px;
+    margin-left: -240px;
+    background: red;
+  }
+  .green {
+    display: block;
+    width: 410px;
+    height: 34px;
+    line-height: 34px;
+    background: green;
+    border: none
+  }
+</style>
+
+<div class="red"><div class="green"></div>
+