Shape tools: review follow-ups from CodeRabbit

- The 45° move assertion compares dx and dy with a small tolerance instead of
  exact equality (the matrix's floats are rounded), keeping the non-zero check
  and the message.
- The engine test helpers say what they do, so the docstring coverage of the
  functions this pull request touches is complete.

crates/engine/src/tests/shapetools.rs

@@ -9,22 +9,26 @@ use vectorcraft_tools::{Mods, PointerEvent, PointerKind, ToolKey};
 
 use crate::*;
 
+/// A new 400 × 400 pt document.
 fn new_doc() -> Session {
     let mut s = Session::new();
     s.execute("file.new", &json!({"width": 400, "height": 400})).unwrap();
     s
 }
 
+/// The view the gestures run in: page coordinates, with Smart Guides off so nothing snaps.
 fn view() -> ViewInfo {
     ViewInfo { smart_guides: false, ..ViewInfo::default() }
 }
 
+/// Feed the pointer events `(kind, x, y)`, all with `mods` held, to the active tool.
 fn gesture(s: &mut Session, mods: Mods, events: &[(PointerKind, f64, f64)]) {
     for (k, x, y) in events {
         s.pointer(&PointerEvent::new(*k, *x, *y).with_mods(mods), view()).unwrap();
     }
 }
 
+/// No modifier held.
 const NONE: Mods = Mods { shift: false, alt: false, cmd: false, ctrl: false, space: false };
 
 /// A 100 × 80 pt rectangle drawn from (100, 100) to (200, 180) with the Rectangle tool, still
@@ -35,14 +39,17 @@ fn drawn_rect(s: &mut Session) -> NodeId {
     *s.doc().unwrap().selection.objects.first().expect("the drawn rectangle is selected")
 }
 
+/// The geometric bounds of `id`.
 fn bounds(s: &Session, id: NodeId) -> Rect {
     s.doc().unwrap().doc.bounds_of(&[id], false).expect("bounds")
 }
 
+/// The angle of `id`'s bounding box, in degrees counter-clockwise: 0 for art square to the page.
 fn angle(s: &Session, id: NodeId) -> f64 {
     s.doc().unwrap().doc.oriented_bounds(&[id], false).map_or(0.0, |b| b.angle)
 }
 
+/// How many undo steps the active document's history holds.
 fn undo_steps(s: &Session) -> usize {
     s.doc().unwrap().history.undo.len()
 }

crates/tools/src/select/tests.rs

@@ -2191,7 +2191,8 @@ fn a_shift_press_on_the_centre_point_moves_or_deselects() {
     let Some(Action::Preview(cmd, v)) = a.last().filter(|a| matches!(a, Action::Preview(c, _) if c == "object.transform")) else { panic!("{a:?}") };
     assert_eq!(cmd, "object.transform");
     let [_, _, _, _, dx, dy] = [0, 1, 2, 3, 4, 5].map(|i| v["matrix"][i].as_f64().unwrap_or_default());
-    assert!(dx != 0.0 && dx == dy, "held to 45°: {dx} {dy}");
+    // A 45° move, within the rounding of the matrix's own floats.
+    assert!(dx != 0.0 && (dx - dy).abs() < 1e-9, "held to 45°: {dx} {dy}");
     assert_eq!(t.pointer(&c, &ev(PointerKind::Up, 170.0, 160.0).with_mods(shift)), vec![Action::Commit]);
     // A Shift-click there takes the object out of the selection on release.
     let mut t = SelectionTool::default();