A moved comment is drawn where it goes while it is dragged

Moving a rectangle, an oval or any other comment showed only its blue
frame going, the shape staying where it was until release. The comment is
now rendered alone once as the drag starts (Document::comment_layer: a
read-only copy with only that comment, no page content) and laid over the
page where it goes; the page beneath keeps showing it where it was until
the move is applied, as a CAD program's move does. Rendering the page
without it would cost a whole page's render at every drag on a heavy plan.
Stamps and Fill & Sign marks keep their own layers. annot: isolate_annotations
for several comments of a page, so a group can be drawn together.

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

crates/engine/src/tests.rs

@@ -3485,6 +3485,44 @@ trailer << /Root 1 0 R >>
     assert!(doc.stamp_preview(0, 1).unwrap().is_none(), "a stamp without an appearance");
 }
 
+/// A moved comment's layer: the comment alone on a transparent page (no page text, no other
+/// comment), drawn as on screen, another app's comment without an appearance too. Preparing it
+/// changes nothing in the document.
+#[test]
+fn comment_layer_draws_the_comment_alone() {
+    let (mut s, id) = session_with(1);
+    s.apply(id, rect_comment(0, [140.0, 40.0, 190.0, 90.0])).unwrap();
+    s.apply(id, rect_comment(0, [20.0, 20.0, 60.0, 60.0])).unwrap();
+    let doc = s.get(id).unwrap();
+    let (generation, dirty) = (doc.edit_generation(), doc.dirty);
+    let layer = doc.comment_layer(0, &[0]).unwrap().expect("a rectangle").render(1.0).unwrap();
+    let px = |p: &pdfcraft_render::RenderedPage, x: u32, y: u32| -> [u8; 4] {
+        p.rgba[(((300 - y) * p.width + x) * 4) as usize..][..4].try_into().unwrap()
+    };
+    assert_eq!(px(&layer, 165, 65), [255, 0, 0, 255], "the rectangle is drawn");
+    let near = |x: u32, y: u32| (137..=193).contains(&x) && (37..=93).contains(&y);
+    let stray: Vec<(u32, u32)> =
+        (1..300).flat_map(|y| (0..200).map(move |x| (x, y))).filter(|&(x, y)| !near(x, y) && px(&layer, x, y) != [0, 0, 0, 0]).collect();
+    assert!(stray.is_empty(), "only the rectangle is on its layer: {:?}", &stray[..stray.len().min(8)]);
+    assert!(doc.comment_layer(0, &[9]).unwrap().is_none(), "no such comment");
+    let both = doc.comment_layer(0, &[0, 1]).unwrap().expect("both").render(1.0).unwrap();
+    assert_eq!((px(&both, 165, 65), px(&both, 40, 40)), ([255, 0, 0, 255], [255, 0, 0, 255]), "moved together, drawn together");
+    let doc = s.get(id).unwrap();
+    assert_eq!((doc.edit_generation(), doc.dirty, doc.can_undo()), (generation, dirty, Some("Add rectangle")), "the document is unchanged");
+
+    let bare = b"%PDF-1.7
+1 0 obj << /Type /Catalog /Pages 2 0 R >> endobj
+2 0 obj << /Type /Pages /Kids [3 0 R] /Count 1 /MediaBox [0 0 200 300] >> endobj
+3 0 obj << /Type /Page /Parent 2 0 R /Annots [4 0 R] >> endobj
+4 0 obj << /Type /Annot /Subtype /Square /Rect [140 40 190 90] /C [1 0 0] >> endobj
+trailer << /Root 1 0 R >>
+%%EOF";
+    let mut s = Session::new();
+    let id = s.open("bare.pdf", None, Arc::new(bare.to_vec()), None).unwrap();
+    let layer = s.get(id).unwrap().comment_layer(0, &[0]).unwrap().expect("drawn as on screen").render(1.0).unwrap();
+    assert!((138..=142).any(|x| px(&layer, x, 65)[..3] == [255, 0, 0]), "the square without an appearance is drawn");
+}
+
 #[test]
 fn flattening_fill_and_sign_keeps_other_comments_fields_and_undoes() {
     let (mut s, id) = session_with(1);