Keep retained masks and reparented drawings alive, and advance collapsed slots

src/widget/position/span.rs

@@ -108,6 +108,7 @@ impl Widget for Span {
             {
                 painter.undraw(child);
                 fixed.px += self.gap;
+                start = shared(fixed, taken, total.leftover, room);
                 continue;
             }
             let from = start;