Artboards: the doc comment names the shared corner the first artboard holds

On an edge two artboards share, the artboard to the right of it or below
it holds a point, except at the end of the edge that lies on that
artboard's bottom or right edge, such as where both artboards' bottom
edges meet. There the first artboard holds it, or the last when
topmost. The doc comment and the test comments now say so, and the test
asserts that corner.

crates/doc/src/lib.rs

@@ -1373,7 +1373,8 @@ impl Document {
     /// inside or on its left or top edge, else the first (the last) that holds it by
     /// [`artboard_holds`]: on its right or bottom edge, or within a millionth of a point outside an
     /// edge. On an edge two artboards share, the artboard to the right of it or below it holds the
-    /// point.
+    /// point, except at the end of the edge that lies on that artboard's bottom or right edge, such
+    /// as the point where both artboards' bottom edges meet; there the first (the last) holds it.
     pub fn artboard_holding(&self, p: Point, topmost: bool) -> Option<usize> {
         let pick = |on: &dyn Fn(Rect) -> bool| {
             let on = |i: &usize| self.artboards.get(*i).is_some_and(|a| on(a.rect));
@@ -1881,9 +1882,13 @@ mod tests {
         assert_eq!(held(&d, 200.001, 50.0), (None, None));
         assert_eq!(held(&d, 50.0, 200.001), (None, None));
         // On an edge two artboards share, the artboard to the right of it or below it holds the
-        // point, in either list order.
+        // point, in either list order, except at the end of the edge that lies on that artboard's
+        // bottom or right edge.
         assert_eq!(held(&d, 100.0, 50.0), (Some(1), Some(1)));
         assert_eq!(held(&d, 50.0, 100.0), (Some(2), Some(2)));
+        // The bottom edges of artboards 0 and 1 meet at (100, 100), which is also artboard 2's top
+        // right corner: the first holds the point, or the last when `topmost`.
+        assert_eq!(held(&d, 100.0, 100.0), (Some(2), Some(0)), "shared corner");
         d.artboards.swap(0, 1);
         assert_eq!(held(&d, 100.0, 50.0), (Some(0), Some(0)));
         // Where two artboards overlap, the topmost holds the point, or the first one when not

crates/engine/src/tests_newdoc.rs

@@ -330,8 +330,8 @@ fn rearrange_lays_out_by_layout_and_order() {
     }
 }
 
-/// Art centered on an artboard's right or bottom edge moves with that artboard, and art on the edge
-/// two artboards share moves with the artboard to the right of it.
+/// Art centered on an artboard's right or bottom edge moves with that artboard, and art centered on
+/// the edge two artboards share, away from its ends, moves with the artboard to the right of it.
 #[test]
 fn rearrange_moves_art_centered_on_an_artboard_edge() {
     let mut s = new_doc(json!({"width": 100, "height": 50, "artboards": 2, "artboardLayout": {"layout": "row", "spacing": 0}}));

crates/engine/src/tests_textimport.rs

@@ -52,8 +52,8 @@ fn crlf_with_returns_removed_places_one_paragraph_per_block_as_area_type() {
 }
 
 /// Placed at a point on an artboard's right or bottom edge, the frame takes that artboard's size less
-/// a 36 pt margin on each side; on the edge two artboards share, it takes the size of the artboard to
-/// the right of it.
+/// a 36 pt margin on each side; on the edge two artboards share, away from its ends, it takes the
+/// size of the artboard to the right of it.
 #[test]
 fn type_placed_on_an_artboard_edge_takes_that_artboards_frame() {
     let mut s = session();

crates/svg/tests/css_rules.rs

@@ -71,7 +71,8 @@ fn rectangle_gives_background_border_and_radius() {
 }
 
 /// An object centered on an artboard's right or bottom edge is positioned from that artboard's
-/// top-left corner; on the edge two artboards share, from the one to the right of it.
+/// top-left corner; on the edge two artboards share, away from its ends, from the one to the right
+/// of it.
 #[test]
 fn position_is_from_the_artboard_whose_edge_holds_the_center() {
     let mut d = doc();