Fix panic when html element has display: table.

components/layout/flow.rs

@@ -295,7 +295,10 @@ pub trait Flow: fmt::Debug + Sync {
     }
 
     /// Marks this flow as the root flow. The default implementation is a no-op.
-    fn mark_as_root(&mut self) {}
+    fn mark_as_root(&mut self) {
+        debug!("called mark_as_root() on a flow of type {:?}", self.class());
+        panic!("called mark_as_root() on an unhandled flow");
+    }
 
     // Note that the following functions are mostly called using static method
     // dispatch, so it's ok to have them in this trait. Plus, they have

components/layout/list_item.rs

@@ -74,6 +74,10 @@ impl Flow for ListItemFlow {
         &mut self.block_flow
     }
 
+    fn as_immutable_block<'a>(&'a self) -> &'a BlockFlow {
+        &self.block_flow
+    }
+
     fn bubble_inline_sizes(&mut self) {
         // The marker contributes no intrinsic inline-size, so…
         self.block_flow.bubble_inline_sizes()

components/layout/multicol.rs

@@ -48,6 +48,10 @@ impl Flow for MulticolFlow {
         &mut self.block_flow
     }
 
+    fn as_immutable_block<'a>(&'a self) -> &'a BlockFlow {
+        &self.block_flow
+    }
+
     fn bubble_inline_sizes(&mut self) {
         // FIXME(SimonSapin) http://dev.w3.org/csswg/css-sizing/#multicol-intrinsic
         self.block_flow.bubble_inline_sizes();

components/layout/table.rs

@@ -210,6 +210,10 @@ impl Flow for TableFlow {
         &self.block_flow
     }
 
+    fn mark_as_root(&mut self) {
+        self.block_flow.mark_as_root();
+    }
+
     fn column_intrinsic_inline_sizes<'a>(&'a mut self) -> &'a mut Vec<ColumnIntrinsicInlineSize> {
         &mut self.column_intrinsic_inline_sizes
     }

components/layout/table_wrapper.rs

@@ -273,6 +273,14 @@ impl Flow for TableWrapperFlow {
         &mut self.block_flow
     }
 
+    fn as_immutable_block<'a>(&'a self) -> &'a BlockFlow {
+        &self.block_flow
+    }
+
+    fn mark_as_root(&mut self) {
+        self.block_flow.mark_as_root();
+    }
+
     fn bubble_inline_sizes(&mut self) {
         // Get the intrinsic column inline-sizes info from the table flow.
         for kid in self.block_flow.base.child_iter() {