Restore stripped whitespace before reflowing text fragments

Fixes #10200.

components/layout/fragment.rs

@@ -1692,22 +1692,39 @@ impl Fragment {
             (&mut SpecificFragmentInfo::ScannedText(ref mut this_info),
              &SpecificFragmentInfo::ScannedText(ref other_info)) => {
                 debug_assert!(util::arc_ptr_eq(&this_info.run, &other_info.run));
-                this_info.range.extend_to(other_info.range_end_including_stripped_whitespace);
+                this_info.range_end_including_stripped_whitespace =
-                this_info.content_size.inline =
+                    other_info.range_end_including_stripped_whitespace;
-                    this_info.run.metrics_for_range(&this_info.range).advance_width;
                 if other_info.requires_line_break_afterward_if_wrapping_on_newlines() {
                     this_info.flags.insert(REQUIRES_LINE_BREAK_AFTERWARD_IF_WRAPPING_ON_NEWLINES);
                 }
                 self.border_padding.inline_end = next_fragment.border_padding.inline_end;
-                self.border_box.size.inline = this_info.content_size.inline +
-                    self.border_padding.inline_start_end();
             }
             _ => panic!("Can only merge two scanned-text fragments!"),
         }
-
+        self.reset_text_range_and_inline_size();
         self.meld_with_next_inline_fragment(&next_fragment);
     }
 
+    /// Restore any whitespace that was stripped from a text fragment, and recompute inline metrics
+    /// if necessary.
+    pub fn reset_text_range_and_inline_size(&mut self) {
+        match &mut self.specific {
+            &mut SpecificFragmentInfo::ScannedText(ref mut info) => {
+                // FIXME (mbrubeck): Do we need to restore leading too?
+                let range_end = info.range_end_including_stripped_whitespace;
+                if info.range.end() == range_end {
+                    return
+                }
+                info.range.extend_to(range_end);
+                info.content_size.inline = info.run.metrics_for_range(&info.range).advance_width;
+                self.border_box.size.inline = info.content_size.inline +
+                    self.border_padding.inline_start_end();
+            }
+            _ => {}
+        }
+    }
+
+
     /// Assigns replaced inline-size, padding, and margins for this fragment only if it is replaced
     /// content per CSS 2.1 § 10.3.2.
     pub fn assign_replaced_inline_size_if_necessary(&mut self, container_inline_size: Au) {

components/layout/inline.rs

@@ -527,6 +527,9 @@ impl LineBreaker {
                        mut fragment: Fragment,
                        flow: &InlineFlow,
                        layout_context: &LayoutContext) {
+        // Undo any whitespace stripping from previous reflows.
+        fragment.reset_text_range_and_inline_size();
+
         // Determine initial placement for the fragment if we need to.
         //
         // Also, determine whether we can legally break the line before, or inside, this fragment.

tests/wpt/mozilla/meta/MANIFEST.json

@@ -2084,6 +2084,30 @@
             "url": "/_mozilla/css/incremental_text_color_a.html"
           }
         ],
+        "css/incremental_trailing_whitespace_a.html": [
+          {
+            "path": "css/incremental_trailing_whitespace_a.html",
+            "references": [
+              [
+                "/_mozilla/css/incremental_trailing_whitespace_ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/css/incremental_trailing_whitespace_a.html"
+          }
+        ],
+        "css/incremental_trailing_whitespace_ref.html": [
+          {
+            "path": "css/incremental_trailing_whitespace_ref.html",
+            "references": [
+              [
+                "/_mozilla/css/incremental_trailing_whitespace_ref.html",
+                "=="
+              ]
+            ],
+            "url": "/_mozilla/css/incremental_trailing_whitespace_ref.html"
+          }
+        ],
         "css/incremental_visibility_a.html": [
           {
             "path": "css/incremental_visibility_a.html",
@@ -8422,6 +8446,30 @@
           "url": "/_mozilla/css/incremental_text_color_a.html"
         }
       ],
+      "css/incremental_trailing_whitespace_a.html": [
+        {
+          "path": "css/incremental_trailing_whitespace_a.html",
+          "references": [
+            [
+              "/_mozilla/css/incremental_trailing_whitespace_ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/css/incremental_trailing_whitespace_a.html"
+        }
+      ],
+      "css/incremental_trailing_whitespace_ref.html": [
+        {
+          "path": "css/incremental_trailing_whitespace_ref.html",
+          "references": [
+            [
+              "/_mozilla/css/incremental_trailing_whitespace_ref.html",
+              "=="
+            ]
+          ],
+          "url": "/_mozilla/css/incremental_trailing_whitespace_ref.html"
+        }
+      ],
       "css/incremental_visibility_a.html": [
         {
           "path": "css/incremental_visibility_a.html",

tests/wpt/mozilla/tests/css/incremental_trailing_whitespace_a.html

@@ -0,0 +1,27 @@
+<!doctype html>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>incremental trailing whitespace test</title>
+    <link rel="match" href="incremental_trailing_whitespace_ref.html">
+    <style>
+      #a {
+        border: 0 solid;
+      }
+      #a.go {
+        border-bottom-width: 4px !important;
+      }
+    </style>
+  </head>
+  <body>
+    <div style="display: inline-block; width: 33.3333%;"></div>
+    <div style="display: inline-block; width: 66.66%;">
+      <div id="a" style="transition: all .1s ease-out;">Hello</div>
+    </div>
+    <script>
+      document.body.offsetWidth; // force layout
+      document.querySelector('#a').classList.add('go');
+      // FIXME (#10245): Wait for the "transitionend" event.
+    </script>
+  </body>
+</html>

tests/wpt/mozilla/tests/css/incremental_trailing_whitespace_ref.html

@@ -0,0 +1,20 @@
+<!doctype html>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>incremental trailing whitespace reference</title>
+    <link rel="match" href="incremental_trailing_whitespace_ref.html">
+    <style>
+      #a {
+        border: 0 solid;
+        border-bottom-width: 4px !important;
+      }
+    </style>
+  </head>
+  <body>
+    <div style="display: inline-block; width: 33.3333%;"></div>
+    <div style="display: inline-block; width: 66.66%;">
+      <div id="a">Hello</div>
+    </div>
+  </body>
+</html>