Auto merge of #29757 - Loirooriol:sequential-context-2020, r=Loirooriol

Lay out floats and handle clearance in layout 2020, but don't flow text around them yet

This is a crude rebase of #27539

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components/layout_2020/positioned.rs

@@ -666,6 +666,7 @@ impl HoistedAbsolutelyPositionedBox {
                 pbm.padding,
                 pbm.border,
                 margin,
+                Length::zero(),
                 CollapsedBlockMargins::zero(),
                 physical_overconstrained,
             )
@@ -808,7 +809,7 @@ fn adjust_static_positions(
 
         let child_fragment_rect = match &child_fragments[original_tree_rank] {
             Fragment::Box(b) => &b.content_rect,
-            Fragment::AbsoluteOrFixedPositioned(_) => continue,
+            Fragment::AbsoluteOrFixedPositioned(_) | Fragment::Float(_) => continue,
             Fragment::Anonymous(a) => &a.rect,
             _ => unreachable!(),
         };