iris: clip ellipsized text at cluster boundaries

docs/PLAN.md

@@ -862,7 +862,10 @@ absolute position back into this same public field; callers animating it own
 stopping their animation when editing begins. Position changes and changes
 among the unwrapped treatments reuse shaping; entering or leaving `Wrap`
 reshapes at the viewport width, while content, font, spans, or density retain
-the normal shape invalidation path.
+the normal shape invalidation path. `Hidden` clips at the exact pixel position
+for continuous scrolling. `Ellipsis` keeps that same continuous position but
+snaps each content-mask edge outward past any shaped cluster or ligature it
+crosses, so a marker never leaves a partial shaped cluster behind.
 An ellipsis marker maps its visually omitted side to one contiguous source
 range. This is exact for ordinary LTR and RTL runs; mixed-bidi text can place
 discontiguous logical ranges on one visual side, which Iris's deliberately