tor-browser

commit ed620cdee7bb063499f54733c34230e437694c1a
parent a9a60b32dcc92e40f6460c8e28a1a072919588bc
Author: Jonathan Watt <jwatt@jwatt.org>
Date:   Tue,  9 Dec 2025 17:22:27 +0000

Bug 2004161. Fix position-area default self-alignment. r=dshin,firefox-style-system-reviewers,layout-reviewers,emilio

https://drafts.csswg.org/css-anchor-position-1/#position-area-alignment

Differential Revision: https://phabricator.services.mozilla.com/D275625

Diffstat:
M
layout/generic/WritingModes.h
 | 
10
++++++++++
M
layout/generic/nsContainerFrame.cpp
 | 
13
++++++++-----
M
servo/ports/geckolib/glue.rs
 | 
20
++++++++++++++++----

3 files changed, 34 insertions(+), 9 deletions(-)
diff --git a/layout/generic/WritingModes.h b/layout/generic/WritingModes.h
@@ -71,6 +71,12 @@ using PhysicalAxes = EnumSet<PhysicalAxis>;
 static constexpr PhysicalAxes kPhysicalAxesBoth{PhysicalAxis::Vertical,
                                                 PhysicalAxis::Horizontal};
 
+inline StyleLogicalAxis ToStyleLogicalAxis(LogicalAxis aLogicalAxis) {
+  return StyleLogicalAxis(aLogicalAxis == LogicalAxis::Block
+                              ? StyleLogicalAxis::Block
+                              : StyleLogicalAxis::Inline);
+}
+
 inline LogicalAxis GetOrthogonalAxis(LogicalAxis aAxis) {
   return aAxis == LogicalAxis::Block ? LogicalAxis::Inline : LogicalAxis::Block;
 }
@@ -474,6 +480,10 @@ class WritingMode {
     mWritingMode = aComputedStyle->WritingMode();
   }
 
+  inline StyleWritingMode ToStyleWritingMode() const {
+    return StyleWritingMode(GetBits());
+  }
+
   /**
    * This function performs fixup for elements with 'unicode-bidi: plaintext',
    * where inline directionality is derived from the Unicode bidi categories
diff --git a/layout/generic/nsContainerFrame.cpp b/layout/generic/nsContainerFrame.cpp
@@ -2550,11 +2550,14 @@ nsContainerFrame::CSSAlignmentForAbsPosChildWithinContainingBlock(
                                                   : StyleAlignFlags::START;
       alignment |= StyleAlignFlags::UNSAFE;
     } else {
-      auto keyword = aLogicalAxis == LogicalAxis::Inline
-                         ? aResolvedPositionArea.first
-                         : aResolvedPositionArea.second;
-      // Use normal position-area alignment
-      Servo_ResolvePositionAreaSelfAlignment(&keyword, &alignment);
+      // Use default position-area self-alignment:
+      // https://drafts.csswg.org/css-anchor-position-1/#position-area-alignment
+      const auto axis = ToStyleLogicalAxis(aLogicalAxis);
+      const auto cbSWM = cbWM.ToStyleWritingMode();
+      const auto selfWM =
+          aChildRI.mFrame->GetWritingMode().ToStyleWritingMode();
+      Servo_ResolvePositionAreaSelfAlignment(&aResolvedPositionArea, axis,
+                                             &cbSWM, &selfWM, &alignment);
     }
   }
 
diff --git a/servo/ports/geckolib/glue.rs b/servo/ports/geckolib/glue.rs
@@ -106,7 +106,7 @@ use style::invalidation::element::relative_selector::{
 };
 use style::invalidation::element::restyle_hints::RestyleHint;
 use style::invalidation::stylesheets::RuleChangeKind;
-use style::logical_geometry::{PhysicalAxis, PhysicalSide, WritingMode};
+use style::logical_geometry::{LogicalAxis, PhysicalAxis, PhysicalSide, WritingMode};
 use style::media_queries::MediaList;
 use style::parser::{Parse, ParserContext};
 use style::properties::declaration_block::PropertyTypedValue;
@@ -165,7 +165,7 @@ use style::values::computed::length_percentage::{
 };
 use style::values::computed::position::{AnchorFunction, PositionArea};
 use style::values::computed::{
-    self, ContentVisibility, Context, PositionAreaKeyword, ToComputedValue,
+    self, ContentVisibility, Context, ToComputedValue,
 };
 use style::values::distance::{ComputeSquaredDistance, SquaredDistance};
 use style::values::generics::color::ColorMixFlags;
@@ -10842,12 +10842,24 @@ pub extern "C" fn Servo_PhysicalizePositionArea(
     *area = area.to_physical(*cb_wm, *self_wm);
 }
 
+/// https://drafts.csswg.org/css-anchor-position-1/#position-area-alignment
 #[no_mangle]
 pub extern "C" fn Servo_ResolvePositionAreaSelfAlignment(
-    area: &PositionAreaKeyword,
+    area: &PositionArea,
+    axis: LogicalAxis,
+    cb_wm: &WritingMode,
+    self_wm: &WritingMode,
     out: &mut AlignFlags,
 ) {
-    let Some(align) = area.to_self_alignment() else {
+    // As well as converting `area` and `axis` to the same form for comparison
+    // this also makes sure `area`'s second keyword explicit (not none).
+    let physical_area = area.to_physical(*cb_wm, *self_wm);
+    let physical_axis = axis.to_physical(*cb_wm);
+    let area_keyword = match physical_axis {
+        PhysicalAxis::Horizontal => physical_area.first,
+        PhysicalAxis::Vertical => physical_area.second,
+    };
+    let Some(align) = area_keyword.to_self_alignment() else {
         debug_assert!(
             false,
             "ResolvePositionAreaSelfAlignment called on {:?}",