Auto merge of #13470 - pcwalton:inline-absolute-hypothetical-baseline, r=notriddle

layout: Improve the interaction between baseline-offset-of-last-line-in-flow logic and inline absolute hypothetical boxes.

See commits for details. These changes place the heart icon on Twitter in the right place.

r? @notriddle

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/13470)
<!-- Reviewable:end -->

5 files changed, 49 insertions, 7 deletions

diff --git a/components/layout/flow.rs b/components/layout/flow.rs
index c8703272fd6..6cdbdeffe30 100644
--- a/components/layout/flow.rs
+++ b/components/layout/flow.rs
@@ -1404,7 +1404,9 @@ impl<'a> ImmutableFlowUtils for &'a Flow {
     fn baseline_offset_of_last_line_box_in_flow(self) -> Option<Au> {
         for kid in base(self).children.iter().rev() {
             if kid.is_inline_flow() {
-                return kid.as_inline().baseline_offset_of_last_line()
+                if let Some(baseline_offset) = kid.as_inline().baseline_offset_of_last_line() {
+                    return Some(baseline_offset)
+                }
             }
             if kid.is_block_like() &&
                     kid.as_block().formatting_context_type() == FormattingContextType::None &&
diff --git a/components/layout/inline.rs b/components/layout/inline.rs
index ec1d223cd36..099d9af14d5 100644
--- a/components/layout/inline.rs
+++ b/components/layout/inline.rs
@@ -1262,13 +1262,16 @@ impl InlineFlow {
     }
 
     pub fn baseline_offset_of_last_line(&self) -> Option<Au> {
-        match self.lines.last() {
-            None => None,
-            Some(ref last_line) => {
-                Some(last_line.bounds.start.b + last_line.bounds.size.block -
-                     last_line.inline_metrics.depth_below_baseline)
+        // Find the last line that doesn't consist entirely of hypothetical boxes.
+        for line in self.lines.iter().rev() {
+            if (line.range.begin().get()..line.range.end().get()).any(|index| {
+                !self.fragments.fragments[index as usize].is_hypothetical()
+            }) {
+                return Some(line.bounds.start.b + line.bounds.size.block -
+                            line.inline_metrics.depth_below_baseline)
             }
         }
+        None
     }
 }
 
@@ -1451,7 +1454,6 @@ impl Flow for InlineFlow {
                                            self.minimum_depth_below_baseline);
         scanner.scan_for_lines(self, layout_context);
 
-
         // Now, go through each line and lay out the fragments inside.
         let line_count = self.lines.len();
         for (line_index, line) in self.lines.iter_mut().enumerate() {
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
index fc7ee02cb96..208db196a24 100644
--- a/tests/wpt/mozilla/meta/MANIFEST.json
+++ b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -2372,6 +2372,18 @@
             "url": "/_mozilla/css/incremental_visibility_a.html"
           }
         ],
+        "css/inline_absolute_hypothetical_baseline_a.html": [
+          {
+            "path": "css/inline_absolute_hypothetical_baseline_a.html",
+            "references": [
+              [
+                "/_mozilla/css/inline_absolute_hypothetical_baseline_ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/css/inline_absolute_hypothetical_baseline_a.html"
+          }
+        ],
         "css/inline_absolute_hypothetical_clip_a.html": [
           {
             "path": "css/inline_absolute_hypothetical_clip_a.html",
@@ -16010,6 +16022,18 @@
           "url": "/_mozilla/css/incremental_visibility_a.html"
         }
       ],
+      "css/inline_absolute_hypothetical_baseline_a.html": [
+        {
+          "path": "css/inline_absolute_hypothetical_baseline_a.html",
+          "references": [
+            [
+              "/_mozilla/css/inline_absolute_hypothetical_baseline_ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/css/inline_absolute_hypothetical_baseline_a.html"
+        }
+      ],
       "css/inline_absolute_hypothetical_clip_a.html": [
         {
           "path": "css/inline_absolute_hypothetical_clip_a.html",
diff --git a/tests/wpt/mozilla/tests/css/inline_absolute_hypothetical_baseline_a.html b/tests/wpt/mozilla/tests/css/inline_absolute_hypothetical_baseline_a.html
new file mode 100644
index 00000000000..c54a2c36990
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/inline_absolute_hypothetical_baseline_a.html
@@ -0,0 +1,10 @@
+<!doctype html>
+<meta charset="utf-8">
+<title></title>
+<link rel="match" href="inline_absolute_hypothetical_baseline_ref.html">
+<div style="display: inline-block">
+    <div>A</div>
+    <span style="position: absolute;">B</span>
+</div>
+C
+D
diff --git a/tests/wpt/mozilla/tests/css/inline_absolute_hypothetical_baseline_ref.html b/tests/wpt/mozilla/tests/css/inline_absolute_hypothetical_baseline_ref.html
new file mode 100644
index 00000000000..e5c686e02a5
--- /dev/null
+++ b/tests/wpt/mozilla/tests/css/inline_absolute_hypothetical_baseline_ref.html
@@ -0,0 +1,4 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<div>A C D</div>
+<div>B</div>