Reorder flex items before skipping already drawn elements

One more fix for flex boxes that will cause painful regressions. Will be fully
fixed when this whole file is finally rewritten…

Fix #1992.

weasyprint/layout/flex.py

@@ -100,6 +100,7 @@ def flex_layout(context, box, bottom_space, skip_stack, containing_block,
         parent_box.width = preferred.flex_max_content_width(
             context, parent_box)
     original_skip_stack = skip_stack
+    children = sorted(children, key=lambda item: item.style['order'])
     if skip_stack is not None:
         (skip, skip_stack), = skip_stack.items()
         if box.style['flex_direction'].endswith('-reverse'):
@@ -263,8 +264,7 @@ def flex_layout(context, box, bottom_space, skip_stack, containing_block,
     line = []
     line_size = 0
     axis_size = getattr(box, axis)
-    for i, child in enumerate(
-            sorted(children, key=lambda item: item.style['order'])):
+    for i, child in enumerate(children):
         if not child.is_flex_item:
             continue
         line_size += child.hypothetical_main_size