servo · bors-servo · Jul 1, 2017 · Jun 30, 2017 · Jun 30, 2017
diff --git a/Cargo.lock b/Cargo.lock
diff --git a/components/layout/Cargo.toml b/components/layout/Cargo.toml
@@ -12,7 +12,7 @@ path = "lib.rs"
 [dependencies]
 app_units = "0.5"
 atomic_refcell = "0.1"
-bitflags = "0.7"
+bitflags = "0.8"
 canvas_traits = {path = "../canvas_traits"}
 euclid = "0.15"
 fnv = "1.0"

diff --git a/components/layout/flow.rs b/components/layout/flow.rs
@@ -1115,6 +1115,28 @@ impl BaseFlow {
         }
     }
 
+    /// Update the 'flags' field when computed styles have changed.
+    ///
+    /// These flags are initially set during flow construction.  They only need to be updated here
+    /// if they are based on properties that can change without triggering `RECONSTRUCT_FLOW`.
+    pub fn update_flags_if_needed(&mut self, style: &ServoComputedValues) {
+        // For absolutely-positioned flows, changes to top/bottom/left/right can cause these flags
+        // to get out of date:
+        if self.restyle_damage.contains(REFLOW_OUT_OF_FLOW) {
+            // Note: We don't need to check whether IS_ABSOLUTELY_POSITIONED has changed, because
+            // changes to the 'position' property trigger flow reconstruction.
+            if self.flags.contains(IS_ABSOLUTELY_POSITIONED) {
+                let logical_position = style.logical_position();
+                self.flags.set(INLINE_POSITION_IS_STATIC,
+                    logical_position.inline_start == LengthOrPercentageOrAuto::Auto &&
+                    logical_position.inline_end == LengthOrPercentageOrAuto::Auto);
+                self.flags.set(BLOCK_POSITION_IS_STATIC,
+                    logical_position.block_start == LengthOrPercentageOrAuto::Auto &&
+                    logical_position.block_end == LengthOrPercentageOrAuto::Auto);
+            }
+        }
+    }
+
     /// Return a new BaseFlow like this one but with the given children list
     pub fn clone_with_children(&self, children: FlowList) -> BaseFlow {
         BaseFlow {
@@ -1361,6 +1383,7 @@ impl<'a> MutableFlowUtils for &'a mut Flow {
     /// calling them individually, since there is no reason not to perform both operations.
     fn repair_style_and_bubble_inline_sizes(self, style: &::StyleArc<ServoComputedValues>) {
         self.repair_style(style);
+        mut_base(self).update_flags_if_needed(style);
         self.bubble_inline_sizes();
     }
 

diff --git a/components/style/servo/restyle_damage.rs b/components/style/servo/restyle_damage.rs
@@ -202,7 +202,7 @@ fn compute_damage(old: &ServoComputedValues, new: &ServoComputedValues) -> Servo
     add_if_not_equal!(old, new, damage,
                       [REPAINT, REPOSITION, STORE_OVERFLOW, BUBBLE_ISIZES, REFLOW_OUT_OF_FLOW,
                        REFLOW, RECONSTRUCT_FLOW], [
-        get_box.float, get_box.display, get_box.position, get_counters.content,
+        get_box.clear, get_box.float, get_box.display, get_box.position, get_counters.content,
         get_counters.counter_reset, get_counters.counter_increment,
         get_inheritedbox._servo_under_display_none,
         get_list.quotes, get_list.list_style_type,

diff --git a/tests/wpt/metadata-css/css21_dev/html4/dynamic-top-change-001.htm.ini b/tests/wpt/metadata-css/css21_dev/html4/dynamic-top-change-001.htm.ini
diff --git a/tests/wpt/mozilla/meta/MANIFEST.json b/tests/wpt/mozilla/meta/MANIFEST.json
@@ -2435,6 +2435,18 @@
      {}
     ]
    ],
+   "css/incremental_position.html": [
+    [
+     "/_mozilla/css/incremental_position.html",
+     [
+      [
+       "/_mozilla/css/incremental_position_ref.html",
+       "=="
+      ]
+     ],
+     {}
+    ]
+   ],
    "css/incremental_text_color_a.html": [
     [
      "/_mozilla/css/incremental_text_color_a.html",
@@ -7911,6 +7923,11 @@
      {}
     ]
    ],
+   "css/incremental_position_ref.html": [
+    [
+     {}
+    ]
+   ],
    "css/incremental_text_color_ref.html": [
     [
      {}
@@ -22808,6 +22825,14 @@
    "3235e9662e37dc723364a1dd39e07157868dd290",
    "support"
   ],
+  "css/incremental_position.html": [
+   "849f3f5ae1df8ecf3bcf3f1723b5a755afeb3316",
+   "reftest"
+  ],
+  "css/incremental_position_ref.html": [
+   "67ab51b29fb015b82bf98942e2e886b3de5ea2cd",
+   "support"
+  ],
   "css/incremental_text_color_a.html": [
    "1846e2691b150cd2eb0b126c6d8f68f47d683fab",
    "reftest"

diff --git a/tests/wpt/mozilla/tests/css/incremental_position.html b/tests/wpt/mozilla/tests/css/incremental_position.html
@@ -0,0 +1,27 @@
+<!doctype html>
+<html class="reftest-wait">
+  <head>
+    <meta charset="utf-8">
+    <title>Incremental absolute position test</title>
+    <link rel="match" href="incremental_position_ref.html">
+    <style>
+      #rect {
+        position: absolute;
+        border: 1px solid red;
+        width: 40px;
+        height: 40px;
+      }
+    </style>
+  </head>
+  <body>
+    <div id="rect"></div>
+    <script>
+      window.onload = function() {
+        document.body.offsetWidth; // force layout
+        var e = document.getElementById("rect");
+        e.style.left = "100px";
+        document.documentElement.classList.remove('reftest-wait');
+      };
+    </script>
+  </body>
+</html>
diff --git a/tests/wpt/mozilla/tests/css/incremental_position_ref.html b/tests/wpt/mozilla/tests/css/incremental_position_ref.html
@@ -0,0 +1,19 @@
+<!doctype html>
+<html>
+  <head>
+    <meta charset="utf-8">
+    <title>Incremental absolute position reference</title>
+    <style>
+      #rect {
+        position: absolute;
+        border: 1px solid red;
+        width: 40px;
+        height: 40px;
+        left: 100px;
+      }
+    </style>
+  </head>
+  <body>
+    <div id="rect"></div>
+  </body>
+</html>