Auto merge of #29835 - mukilan:fix-double-borrow-in-hoisted, r=mrobinson

Clear PositioningContext for speculative layouts

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Developed in collaboration with @mrobinson

`try_layout` is used for laying out absolutely positioned descendants multiple times when min/max-{width, height} properties are set. When the same PositioningContext instance is used between successive attempts without clearing the accumulated descendants, we will generate multiple fragments which reference the same box, which then will lead to a double borrow error when layout is performed in parallel.

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- [ ] These changes fix #___ (GitHub issue number if applicable)

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- [x] These changes do not require tests because crash testing is currently not working in servo. New tests will be added once #29832 is fixed.

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components/layout_2020/positioned.rs

@@ -379,6 +379,14 @@ impl PositioningContext {
             )
         }
     }
+
+    pub(crate) fn clear(&mut self) {
+        self.for_nearest_containing_block_for_all_descendants
+            .clear();
+        self.for_nearest_positioned_ancestor
+            .as_mut()
+            .map(|v| v.clear());
+    }
 }
 
 impl HoistedAbsolutelyPositionedBox {
@@ -564,6 +572,13 @@ impl HoistedAbsolutelyPositionedBox {
                             "Mixed writing modes are not supported yet"
                         );
                         let dummy_tree_rank = 0;
+
+                        // Clear the context since we will lay out the same descendants
+                        // more than once. Otherwise, absolute descendants will create
+                        // multiple fragments which could later lead to double-borrow
+                        // errors.
+                        positioning_context.clear();
+
                         let independent_layout = non_replaced.layout(
                             layout_context,
                             &mut positioning_context,