iris: keep text overflow masks horizontal

docs/PLAN.md

@@ -862,10 +862,12 @@ 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. `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.
+the normal shape invalidation path. Overflow masks only the horizontal axis;
+vertical ink such as descenders and accents remains untouched. `Hidden` clips
+at the exact horizontal 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