fix(comments): the eraser rubs exactly its preview, at any zoom or view rotation

From review of #3749: the user-space radius had a 1-unit floor, so at a
high zoom the eraser rubbed past its preview; and zoom was read from the
screen rectangle's width, which is the page's height when the view is
turned a quarter. PageXform::zoom reads the side the page's width is on;
the eraser and the drawing preview use it, and the floor is gone. A page
with no size on screen erases nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/canvas.rs

@@ -2121,6 +2121,13 @@ pub struct PageXform {
 }
 
 impl PageXform {
+    /// Screen pixels per page point. The view's rotation swaps the rectangle's sides, so a page
+    /// turned a quarter shows its width down the screen.
+    pub(crate) fn zoom(&self) -> f32 {
+        let across = if self.rot % 180 == 90 { self.rect.height() } else { self.rect.width() };
+        across / self.pw.max(1.0)
+    }
+
     /// Draw an image in the PDF user-space `rect` as a custom stamp's appearance draws it: turned
     /// back by `turn` (the page /Rotate it was placed for), so with the page's own rotation it
     /// reads upright as displayed.
@@ -5469,6 +5476,18 @@ mod tests {
         assert_eq!(badges.len(), 1);
     }
 
+    #[test]
+    fn page_zoom_reads_the_side_the_page_width_is_on() {
+        // A 200 x 100 pt page at 2 pixels per point: 400 x 200 on screen, or 200 x 400 turned.
+        let xf = |rot: u16, size: Vec2| PageXform { rect: Rect::from_min_size(Pos2::ZERO, size), rot, pw: 200.0, ph: 100.0 };
+        for rot in [0, 180] {
+            assert_eq!(xf(rot, vec2(400.0, 200.0)).zoom(), 2.0, "{rot}");
+        }
+        for rot in [90, 270] {
+            assert_eq!(xf(rot, vec2(200.0, 400.0)).zoom(), 2.0, "{rot}: the width runs down the screen");
+        }
+    }
+
     #[test]
     fn snapshot_tabs_read_like_their_source_plus_the_suffix() {
         let snap = Snapshot { kind: SnapshotKind::SignedVersion, revision: 1, from: "report.pdf".into() };

crates/ui-egui/src/comments.rs

@@ -1260,7 +1260,7 @@ fn paint_gesture(painter: &egui::Painter, cx: &PageCx<'_>, view: &DocView) {
 fn paint_draw(painter: &egui::Painter, cx: &PageCx<'_>, tool: &CommentTool, points: &[[f64; 2]]) {
     {
         let style = cx.prefs.style(*tool);
-        let zoom = cx.xf.rect.width() / cx.xf.pw.max(1.0);
+        let zoom = cx.xf.zoom();
         let stroke = Stroke::new((style.width as f32 * zoom).max(1.0), color32(style.color));
         let pts: Vec<Pos2> = points.iter().map(|p| cx.to_screen(*p)).collect();
         match tool {
@@ -1346,8 +1346,13 @@ fn eraser_radius_px(width: f64) -> f64 {
 /// Eraser: rub out drawings along `path` (user space); one undo step for all of them. It rubs
 /// as wide on screen as its preview, whatever the zoom.
 fn erase(cx: &PageCx<'_>, path: &[[f64; 2]]) -> Option<Edit> {
-    let zoom = (cx.xf.rect.width() / cx.xf.pw.max(1.0)) as f64;
-    let radius = (eraser_radius_px(cx.prefs.style(CommentTool::Eraser).width) / zoom).max(1.0);
+    let zoom = f64::from(cx.xf.zoom());
+    // A page with no size on screen has nothing to rub out (and no finite radius).
+    if !(zoom.is_finite() && zoom > 0.0) {
+        return None;
+    }
+    // Exactly the preview's width: no floor, which at a high zoom rubbed past it.
+    let radius = eraser_radius_px(cx.prefs.style(CommentTool::Eraser).width) / zoom;
     let b = path.iter().fold([f64::MAX, f64::MAX, f64::MIN, f64::MIN], |r, p| [r[0].min(p[0]), r[1].min(p[1]), r[2].max(p[0]), r[3].max(p[1])]);
     let mut hits: Vec<usize> = cx
         .comments()