Auto merge of #17581 - mbrubeck:incremental, r=pcwalton

Fix incremental layout bugs for absolute and cleared elements

Fixes #17307.  <del>There were two underlying bugs:</del>

* <del>Nodes were not marked dirty when their style attribute changed.</del>

* BaseFlow flags set during flow construction could get out of sync with the element's style if repair_style was called.

---

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #17307
- [x] There are tests for these changes

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2 files changed, 24 insertions, 1 deletions

diff --git a/components/layout/Cargo.toml b/components/layout/Cargo.toml
index 6b1ad9d2e6d..e3e2c76c2f4 100644
--- 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
index f87fe4c9ef8..7d734bd1343 100644
--- 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+        // to get out of date: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();
     }