diff --git a/TODO.txt b/TODO.txt
index bce9015..7a8e120 100644
--- a/TODO.txt
+++ b/TODO.txt
@@ -22,3 +22,7 @@ Performance:
   generalize interface to some traversable so the caller decides what
   structure to use (e.g. tree)
 - Use Vector instead of List where sensible
+- Consider viewport alternative combinator that takes a hash of the data
+  to be rendered, caches the hash when it renders, and only re-renders
+  when it gets a new hash (and the hash goes in the viewport state that
+  gets stored in the RenderState)