Auto merge of #16881 - mbrubeck:absolute_root, r=stshine

Fix containing block size for absolute root element

The root element doesn't have a containing block, so use the viewport size instead of `self.base.absolute_cb`.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #16248 (github issue number if applicable).
- [x] There are tests for these changes

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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 {