Auto merge of #29897 - Loirooriol:clearance-prevents-margin-collapse, r=mrobinson

Prevent margins from collapsing through when separated by clearance

This fixes #29884 and improves #29896.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #29884 and improve #29896
- [X] There are tests for these changes

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components/layout_2020/flow/mod.rs

@@ -657,6 +657,7 @@ fn layout_in_flow_non_replaced_block_level(
                 collapsible_margins_in_children.collapsed_through &&
                     block_is_same_formatting_context &&
                     pbm.padding_border_sums.block == Length::zero() &&
+                    clearance == Length::zero() &&
                     block_size.auto_is(|| Length::zero()) == Length::zero() &&
                     min_box_size.block == Length::zero();
         },