Shape Builder: Pick Color From Artwork takes the look where the drag starts, else where it ends

With Pick Color From Artwork, a Shape Builder merge took the look of
the front-most object over the first covered region the drag touched,
and the current fill when the drag touched only areas no object
covers. The reference app's combine page gives the look of the object
where the drag starts, else of the object where it ends, else of the
topmost selected object.

shapeBuilder.merge now follows that rule: the merged shape takes the
appearance, opacity and blending mode of the front-most of the objects
it is given (the selection by default) under the first point, else of
the one under the last point, else of the front-most of those objects.
A drag from a hole across a gray bar into a red square now gives a red
shape, where it gave the bar's look. A click in an area no selected
object covers, such as a hole that gray bars enclose or a triangle
that lines enclose, takes the front-most selected object's look
instead of the current fill. Pick Color From Color Swatches works as
before. The tool already sends the press point first and the release
point last.

Docs: docs/mcp.md, docs/target-app-parity.md.

crates/engine/src/cmd/buildcmds.rs

@@ -39,7 +39,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Shape Builder",
             [],
             None,
-            "{ids?|id?: [id…] (default: selection), points: [[x,y]…] (drag path; one point = click), erase?: bool (erase mode, Alt: deletes the regions touched, and the pieces of open paths that cut regions and the edges (outline pieces between the points where paths meet; see shapeBuilder.regions) within tolerance; a deleted edge opens its path there, and edges bounding a deleted region go with it), tolerance?: pt (2, erase only), gap?: pt (0–72; 0: Gap Detection off), openFilledAsClosed?: true (Consider Open Filled Paths as Closed: a filled open path bounds a region as if an invisible edge closed it; false: it cuts the regions it crosses like an unfilled one), pickColorFrom?: \"artwork\" (default: the merged shape takes the fill of the front-most object over the first region the drag touches that an object covers; the current fill when only lines enclose every region touched) | \"swatches\" (the current fill), fill?: \"#rrggbb\" | {color|swatch|gradient|none} (overrides pickColorFrom)} → {ids, merged, regions, lines, edges} (counts deleted or merged)",
+            "{ids?|id?: [id…] (default: selection), points: [[x,y]…] (drag path; one point = click), erase?: bool (erase mode, Alt: deletes the regions touched, and the pieces of open paths that cut regions and the edges (outline pieces between the points where paths meet; see shapeBuilder.regions) within tolerance; a deleted edge opens its path there, and edges bounding a deleted region go with it), tolerance?: pt (2, erase only), gap?: pt (0–72; 0: Gap Detection off), openFilledAsClosed?: true (Consider Open Filled Paths as Closed: a filled open path bounds a region as if an invisible edge closed it; false: it cuts the regions it crosses like an unfilled one), pickColorFrom?: \"artwork\" (default: the merged shape takes the look of the front-most object under the first point, else under the last point, else of the front-most object) | \"swatches\" (the current fill, with the rest of the look of the front-most object over the first region the drag touches that an object covers, else of the front-most object), fill?: \"#rrggbb\" | {color|swatch|gradient|none} (overrides pickColorFrom)} → {ids, merged, regions, lines, edges} (counts deleted or merged)",
             has_selection_or_ids,
             sb_merge
         ),
@@ -261,9 +261,8 @@ fn sb_merge(s: &mut Session, p: &Value) -> Result<Value> {
             _ => return Err(bad(C, format!("pickColorFrom is \"artwork\" or \"swatches\", not {v}"))),
         },
     };
-    let current_fill = s.paint.fill.clone();
     let fill = match fill_param(s, p, C)? {
-        None if swatches => Some(current_fill.clone()),
+        None if swatches => Some(s.paint.fill.clone()),
         f => f,
     };
     let roots = root_ids(s, p)?;
@@ -289,11 +288,14 @@ fn sb_merge(s: &mut Session, p: &Value) -> Result<Value> {
         let picked: Vec<&po::Region> = touched.regions.iter().filter_map(|&i| map.regions.get(i).map(|r| &r.0)).collect();
         let faces = po::FaceMerger::with_gap(&map.shapes, gap_param(p));
         let union = if picked.is_empty() { PathData::default() } else { faces.merge(&picked) };
-        // The merged shape takes the look of the front-most object over the first region, or (for an
-        // area only lines enclose) of the front-most object and the current fill.
-        let covered = picked.iter().find_map(|r| r.top());
+        // Pick Color From Artwork: the merged shape takes the look of the front-most object under the
+        // drag's first point, else under its last point, else of the front-most object. Color
+        // Swatches: the current fill, with the rest of the look of the front-most object over the
+        // first region the drag touched that an object covers, else of the front-most object.
+        let top_at = |q: &Point| map.region_at(*q).and_then(|k| map.regions.get(k)).and_then(|(r, ..)| r.top());
+        let covered =
+            if swatches { picked.iter().find_map(|r| r.top()) } else { pts.first().and_then(top_at).or_else(|| pts.last().and_then(top_at)) };
         let style_src = covered.or(map.shapes.len().checked_sub(1));
-        let merged_fill = fill.or_else(|| covered.is_none().then_some(current_fill));
         let mut new_nodes: Vec<Node> = vec![];
         let mut merged_id = None;
         for (i, (sh, leaf)) in map.shapes.iter().zip(&leaves).enumerate() {
@@ -346,7 +348,7 @@ fn sb_merge(s: &mut Session, p: &Value) -> Result<Value> {
             }
             if !erase && !union.is_empty() && Some(i) == style_src {
                 let mut m = shape_node(d, union.clone(), Some(leaf));
-                if let Some(f) = &merged_fill {
+                if let Some(f) = &fill {
                     m.appearance.set_fill(f.clone());
                 }
                 merged_id = Some(m.id);