Auto merge of #12593 - adamncasey:table-row-cell-height, r=pcwalton

Fix table vertical alignment (middle, bottom)

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Fixes table cell vertical alignment (middle, bottom, not yet baseline) when the row contains cells of differing heights.

Moved the work done earlier by @notriddle into a separate public function on TableCellFlow. This function is then called by TableRowFlow once the cell's block size has been calculator.

---

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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 #12531 (github issue number if applicable).

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- [X] There are tests for these changes

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components/layout/table_cell.rs

@@ -80,6 +80,10 @@ impl TableCellFlow {
         if !flow::base(self).restyle_damage.contains(REFLOW) {
             return;
         }
+    }
+
+    /// Position this cell's children according to vertical-align.
+    pub fn valign_children(&mut self) {
         // Note to the reader: this code has been tested with negative margins.
         // We end up with a "end" that's before the "start," but the math still works out.
         let first_start = flow::base(self).children.front().map(|kid| {
@@ -106,6 +110,9 @@ impl TableCellFlow {
             let self_size = flow::base(self).position.size.block -
                 self.block_flow.fragment.border_padding.block_start_end();
             let kids_self_gap = self_size - kids_size;
+
+            // This offset should also account for vertical_align::T::baseline.
+            // Need max cell ascent from the first row of this cell.
             let offset = match self.block_flow.fragment.style().get_box().vertical_align {
                 vertical_align::T::middle => kids_self_gap / 2,
                 vertical_align::T::bottom => kids_self_gap,

components/layout/table_row.rs

@@ -162,6 +162,9 @@ impl TableRowFlow {
             // Assign the child's block size.
             child_table_cell.block_flow.base.position.size.block = block_size;
 
+            // Now we know the cell height, vertical align the cell's children.
+            child_table_cell.valign_children();
+
             // Write in the size of the relative containing block for children. (This information
             // is also needed to handle RTL.)
             child_table_cell.block_flow.base.early_absolute_position_info =