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
+the normal shape invalidation path. Overflow masks only the horizontal axis;
-for continuous scrolling. `Ellipsis` keeps that same continuous position but
+vertical ink such as descenders and accents remains untouched. `Hidden` clips
-snaps each content-mask edge outward past any shaped cluster or ligature it
+at the exact horizontal pixel position for continuous scrolling. `Ellipsis`
-crosses, so a marker never leaves a partial shaped cluster behind.
+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

iris/src/widget/text/mod.rs

@@ -354,11 +354,12 @@ impl TextView {
         } else {
             (self.overflow.content_end - self.overflow.content_start).max(0.0)
         };
-        let mask = vec2(content_width, self.overflow.viewport.y)
+        let horizontal_mask = vec2(content_width, self.overflow.viewport.y)
             .align(Align::TOP_LEFT)
             .offset(vec2(self.overflow.content_start, 0.0))
             .within(&viewport)
             .within(&painter.region());
+        let mask = UiRegion::new(horizontal_mask.x, UiSpan::FULL);
         painter.set_mask(mask);
         let origin = tex
             .size
@@ -656,6 +657,24 @@ mod tests {
         assert_eq!(rsc.ui.widgets[text].view.overflow.trailing_marker, 0.0);
     }
 
+    #[test]
+    fn overflow_masks_only_the_horizontal_axis() {
+        let mut rsc = TestRsc { ui: Ui::default() };
+        let text = wtext("glyphs jumping")
+            .overflow(TextOverflow::Hidden)
+            .size(18)
+            .add(&mut rsc);
+        let root = text.width(abs(50)).add_strong(&mut rsc).any();
+        let mut render = UiRenderState::new();
+        render.resize((800.0, 600.0));
+        render.update(&root, &mut rsc);
+
+        let mask = rsc.ui.masks[render.active[&text.id()].own_mask.idx()];
+        let region = render.primitives.instance(mask.primitive).region;
+        assert_eq!(region.y, UiSpan::FULL);
+        assert_ne!(region.x, UiSpan::FULL);
+    }
+
     #[test]
     fn selecting_an_ellipsis_selects_the_omitted_source_text() {
         let mut rsc = TestRsc { ui: Ui::default() };