iris: process dirty widgets one at a time

docs/LAYOUT.md

@@ -141,11 +141,12 @@ which is correct always, just not free.
 
 **Size invalidation travels upward before drawing; drawing itself travels only
 downward.** Every active widget retains the direct children whose size it read
-through `DrawResult::size()` or `Painter::known_len`. Before a frame draws,
-`redraw_updates` follows only those dependency edges from each dirty child and
-marks the affected ancestors dirty. It then selects the highest dirty roots and
-draws them top-down. Drawing never synchronously invalidates or invokes a
-parent, so there is no layout recursion and no provisional child draw on a
+through `DrawResult::size()` or `Painter::known_len`. `redraw_updates` takes
+one id from the dirty set, follows only those dependency edges upward and marks
+that path dirty, then redraws its highest already-dirty ancestor. Drawing that
+ancestor consumes the marks of every dirty descendant it reaches; the loop
+then takes whatever remains. Drawing never synchronously invalidates or invokes
+a parent, so there is no layout recursion and no provisional child draw on a
 different layer.
 
 An exact `size_hint` stops propagation when both axes still equal the retained