Fix containing block size for absolute root element

Fixes #16248.

components/layout/block.rs

@@ -657,7 +657,7 @@ impl BlockFlow {
     pub fn containing_block_size(&self, viewport_size: &Size2D<Au>, descendant: OpaqueFlow)
                                  -> LogicalSize<Au> {
         debug_assert!(self.base.flags.contains(IS_ABSOLUTELY_POSITIONED));
-        if self.is_fixed() {
+        if self.is_fixed() || self.is_root() {
             // Initial containing block is the CB for the root
             LogicalSize::from_physical(self.base.writing_mode, *viewport_size)
         } else {