Auto merge of #15156 - mbrubeck:border-collapse, r=notriddle

WIP: Fix missing borders in tables with border-collapse

This fixes a pair of bugs that caused some borders to be ignored by the border-collapse code. r? @notriddle

I haven't done a full test run with this change yet. After a `try` build I will update test expectations and/or add a new test.

---

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #14834 (github issue number if applicable).

- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/layout/table.rs

@@ -681,13 +681,18 @@ fn perform_border_collapse_for_row(child_table_row: &mut TableRowFlow,
     }
 
     // Compute block-start borders.
-    match previous_block_borders {
-        PreviousBlockCollapsedBorders::FromPreviousRow(previous_block_borders) => {
-            child_table_row.final_collapsed_borders.block_start = previous_block_borders
-        }
-        PreviousBlockCollapsedBorders::FromTable(collapsed_border) => {
-            child_table_row.final_collapsed_borders.block_start =
-                vec![collapsed_border; child_table_row.block_flow.base.children.len()]
+    let block_start_borders = &mut child_table_row.final_collapsed_borders.block_start;
+    *block_start_borders = child_table_row.preliminary_collapsed_borders.block_start.clone();
+    for (i, this_border) in block_start_borders.iter_mut().enumerate() {
+        match previous_block_borders {
+            PreviousBlockCollapsedBorders::FromPreviousRow(ref previous_block_borders) => {
+                if previous_block_borders.len() > i {
+                    this_border.combine(&previous_block_borders[i]);
+                }
+            }
+            PreviousBlockCollapsedBorders::FromTable(table_border) => {
+                this_border.combine(&table_border);
+            }
         }
     }
 

components/layout/table_row.rs

@@ -927,7 +927,7 @@ fn perform_inline_direction_border_collapse_for_row(
         let first_inline_border = &mut preliminary_collapsed_borders.inline[0];
         first_inline_border.combine(
             &CollapsedBorder::inline_start(&*child_table_cell.block_flow.fragment.style,
-                                           CollapsedBorderProvenance::FromPreviousTableCell));
+                                           CollapsedBorderProvenance::FromNextTableCell));
     }
 
     let inline_collapsed_border = preliminary_collapsed_borders.inline.push_or_set(

tests/wpt/metadata-css/css21_dev/html4/border-conflict-element-001c.htm.ini

@@ -1,3 +0,0 @@
-[border-conflict-element-001c.htm]
-  type: reftest
-  expected: FAIL