Dimension tool: both ends of a dimension follow the art it measures

A linear dimension clicked from an anchor to a point that is not an
anchor (a point on a segment, or a point a Smart Guide gave) followed
the art at the anchor end only, so scaling the art turned it into a
long slanted dimension.

- A point on a segment is stored by its curve parameter on that
  segment, and stays on it when the segment's anchors or handles are
  edited.
- Any other point is stored against anchors (and, for a path whose
  anchors lie on one line, handles) of the path under it, or of the
  path at the other end, and moves, scales, turns and reflects with
  that path. Beside a path whose anchors and handles all lie on one
  line, such as a straight line, it keeps its distance along and
  across the path: a reflection moves it as a half turn does, and a
  scale that is not uniform moves it as a uniform scale does.
- When linked art is reflected, a linear dimension's line goes to the
  reflected side, except for art whose anchors and handles lie on one
  line.
- Deleting the art, changing its anchor count or whether it is closed,
  turning an angle's arm into a curve, or making a measured circle or
  arc no longer circular leaves the dimension where it was, no longer
  linked, instead of removing it. Undo links it again.

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

crates/engine/src/cmd/dimension.rs

@@ -16,7 +16,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Create Dimension",
             [],
             None,
-            "{mode?: linear|angular|radius|diameter, a?: [x,y], b?: [x,y], vertex?: [x,y], source?: {kind: segment, edge: {node,subpath,segment}}|{kind: angle, first: edge, second: edge}|{kind: circle, node}|{kind: arc, edge}|{kind: points, a?, b?, vertex?: {node,subpath,anchor}}, orientation?: aligned|horizontal|vertical (linear only: the distance between the points, or only its horizontal or vertical part; default aligned), offset?: pt (the line's distance from the first point: along the left normal of a → b when aligned, downward when horizontal, leftward when vertical; negative goes the other way), options?: {units: in|mm|pt|pc|px|cm|m|ft|yd, precision: 0-6, scale: drawing:real, hideUnits, lineWeight, lineType: solid|dashed|dotted, lineColor: [r,g,b], arrowStyle: filled|open|tick|none, arrowScale: %, extensionWeight, extensionType, extensionColor, extensionOffset, extensionLength, hideExtensions, fontFamily, fontStyle, fontSize, textColor, textPosition: away|between, textAlignment: left|center|right, textOffset}} → {id,label}; linked measurements follow source edits",
+            "{mode?: linear|angular|radius|diameter, a?: [x,y], b?: [x,y], vertex?: [x,y], source?: {kind: segment, edge: {node,subpath,segment}}|{kind: angle, first: edge, second: edge}|{kind: circle, node}|{kind: arc, edge}|{kind: points, a?, b?, vertex?: {node,subpath,anchor}, freeA?, freeB?: a point that is not an anchor, {kind: onSegment, edge: {node,subpath,segment}, t: 0..1 (curve parameter)}|{kind: anchors, node, anchors: [{subpath,anchor,part?: point|in|out (the anchor, default, or its in or out handle),weight}] (2 or 3 points of one path whose weights add up to 1), across?: (2 anchors only: times the first-to-second anchor vector (x,y) turned to (-y,x), which points down when the vector points right)} (an end is an anchor or a free point, not both)}, orientation?: aligned|horizontal|vertical (linear only: the distance between the points, or only its horizontal or vertical part; default aligned), offset?: pt (the line's distance from the first point: along the left normal of a → b when aligned, downward when horizontal, leftward when vertical; negative goes the other way), options?: {units: in|mm|pt|pc|px|cm|m|ft|yd, precision: 0-6, scale: drawing:real, hideUnits, lineWeight, lineType: solid|dashed|dotted, lineColor: [r,g,b], arrowStyle: filled|open|tick|none, arrowScale: %, extensionWeight, extensionType, extensionColor, extensionOffset, extensionLength, hideExtensions, fontFamily, fontStyle, fontSize, textColor, textPosition: away|between, textAlignment: left|center|right, textOffset}} → {id,label}; linked measurements follow source edits",
             has_doc,
             create
         ),
@@ -231,9 +231,10 @@ fn apply_to_all(s: &mut Session, p: &Value) -> Result<Value> {
 fn replace(doc: &mut Document, id: NodeId, dim: Dimension) -> Result<()> {
     let old: Vec<NodeId> = doc.node(id).and_then(Node::children).map(|ch| ch.iter().map(|n| n.id).collect()).unwrap_or_default();
     let children = artwork(doc, &dim, &old)?;
+    let name = format!("Dimension · {}", dim.label().map_err(error)?);
     if let Some(n) = doc.node_mut(id) {
         n.kind = NodeKind::Group { children, clip: false };
-        n.name = Some(format!("Dimension · {}", dim.label().map_err(error)?));
+        n.name = Some(name);
         n.extras_mut().dimension = Some(Box::new(dim));
     }
     Ok(())
@@ -272,7 +273,8 @@ impl EndedLink {
 
 /// The [`reached`] dimensions become plain groups, whose art does not redraw or follow what it
 /// measured. A command that replaces art calls this before it changes anything, since replacing
-/// the art a dimension measures would remove the dimension, and then [`restore_unchanged`].
+/// the art a dimension measures would otherwise leave the dimension unlinked, and then
+/// [`restore_unchanged`].
 pub(crate) fn end_links(doc: &mut Document, roots: &[NodeId]) -> Vec<EndedLink> {
     let mut ended = Vec::new();
     for id in reached(doc, roots) {
@@ -315,18 +317,37 @@ pub(crate) fn refresh(before: &Document, doc: &mut Document) -> Result<bool> {
         let mut next = old.clone();
         let moved = next.moved.take();
         if next.source.as_ref().is_some_and(|s| s.topology_changed(before, doc)) || next.resolve(doc).is_err() {
-            // A deleted/degenerate source has no valid measurement. Remove its annotation in
-            // this same undo step instead of keeping a silently wrong number.
-            doc.remove(id)?;
+            // The art it measured was deleted, or rebuilt with other anchors, or no longer fits
+            // (an ellipse that was a circle): it stays where it was, no longer linked, in this
+            // same undo step, rather than attaching to a different edge.
+            let mut unlinked = old.clone();
+            unlinked.source = None;
+            unlinked.moved = None;
+            if let Some(v) = moved {
+                let mut shifted = unlinked.clone();
+                (shifted.a, shifted.b, shifted.vertex) = (shifted.a + v, shifted.b + v, shifted.vertex + v);
+                if shifted.validate().is_ok() {
+                    unlinked = shifted;
+                }
+            }
+            if replace(doc, id, unlinked).is_err()
+                && let Some(n) = doc.node_mut(id)
+            {
+                // Its recipe cannot be drawn again: its art stays as it is, as a plain group.
+                n.extras_mut().dimension = None;
+            }
             changed = true;
             continue;
         }
         next.keep_gap(&old);
+        if let Some(xf) = next.source.as_ref().and_then(|s| s.reflection(before, doc)) {
+            next.mirror_line(&old, xf * old.handle());
+        }
         // Moved on its own (the art it measures stayed put): it keeps its new place.
         if let Some(v) = moved
             && (next.a, next.b, next.vertex) == (old.a, old.b, old.vertex)
         {
-            next.place(next.handle() + v);
+            next.place(old.handle() + v);
             if next.resolve(doc).is_err() {
                 next = old.clone();
                 next.moved = None;

crates/engine/src/cmd/expand.rs

@@ -242,7 +242,7 @@ fn expand_steps(s: &mut Session, p: &Value) -> Result<Value> {
     let to = GradientTo::parse(p)?;
     let from = s.doc()?.history.undo.len();
     let mut e = Expandable::of(&s.doc()?.doc, &selected_roots(s)?);
-    // Before anything expands: outlining the art a dimension measures would remove the dimension.
+    // Before anything expands: outlining the art a dimension measures would leave the dimension unlinked.
     let before = s.doc()?.doc.clone();
     let mut ended = vec![];
     end_dimension_links(s, &mut ended)?;