Add TODO item
diff --git a/src/hb-ot-layout-gsubgpos-private.hh b/src/hb-ot-layout-gsubgpos-private.hh
index 244c395..005ed21 100644
--- a/src/hb-ot-layout-gsubgpos-private.hh
+++ b/src/hb-ot-layout-gsubgpos-private.hh
@@ -996,7 +996,29 @@
if (!delta)
continue;
- /* Recursed lookup changed buffer len. Adjust. */
+ /* Recursed lookup changed buffer len. Adjust.
+ *
+ * TODO:
+ *
+ * Right now, if buffer length increased by n, we assume n new glyphs
+ * were added right after the current position, and if buffer length
+ * was decreased by n, we assume n match positions after the current
+ * one where removed. The former (buffer length increased) case is
+ * fine, but the decrease case can be improved in at least two ways,
+ * both of which are significant:
+ *
+ * - If recursed-to lookup is MultipleSubst and buffer length
+ * decreased, then it's current match position that was deleted,
+ * NOT the one after it.
+ *
+ * - If buffer length was decreased by n, it does not necessarily
+ * mean that n match positions where removed, as there might
+ * have been marks and default-ignorables in the sequence. We
+ * should instead drop match positions between current-position
+ * and current-position + n instead.
+ *
+ * It should be possible to construct tests for both of these cases.
+ */
end += delta;
if (end < int (match_positions[idx]))