Correct negative margins in centered table cells

Fixes @AelitaBot queue viewer page

components/layout/table_cell.rs

@@ -80,8 +80,12 @@ impl TableCellFlow {
         if !flow::base(self).restyle_damage.contains(REFLOW) {
             return;
         }
+        // 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| {
+            let kid_base = flow::base(kid);
             flow::base(kid).position.start.b
+                - kid_base.collapsible_margins.block_start_margin_for_noncollapsible_context()
         });
         if let Some(mut first_start) = first_start {
             let mut last_end = first_start;