Auto merge of #12873 - shinglyu:abs-margin-patch, r=notriddle,emilio

Fix absolute-flow's auto positioning

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If an absolute positioned flow has no top, bottom, left, right property, its hypothetical box position should be the margin-end of its previous sibling, not the border-end.

---
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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 #12676 (github issue number if applicable).

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- [x] There are tests for these changes

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components/layout/block.rs

@@ -817,11 +817,14 @@ impl BlockFlow {
             for (child_index, kid) in self.base.child_iter_mut().enumerate() {
                 if flow::base(kid).flags.contains(IS_ABSOLUTELY_POSITIONED) {
                     // Assume that the *hypothetical box* for an absolute flow starts immediately
-                    // after the block-end border edge of the previous flow.
+                    // after the margin-end border edge of the previous flow.
                     if flow::base(kid).flags.contains(BLOCK_POSITION_IS_STATIC) {
+                        let previous_bottom_margin = margin_collapse_info.current_float_ceiling();
+
                         flow::mut_base(kid).position.start.b = cur_b +
                             flow::base(kid).collapsible_margins
-                                           .block_start_margin_for_noncollapsible_context()
+                                           .block_start_margin_for_noncollapsible_context() +
+                            previous_bottom_margin
                     }
                     kid.place_float_if_applicable();
                     if !flow::base(kid).flags.is_float() {