Measure tool: Shift keeps the end on the pointer's step after the grid rounds it

With a Constrain Angle other than 0, the grid point a Shift-drag's end
rounds to can lie nearer another 45-degree step, and the measurement then
reported that step. A Shift-drag from (0, 0) to (5, -3) with a 30-degree
Constrain Angle and a 9 pt grid measured -15 degrees instead of 30. The
end is now turned back onto the step nearest the pointer's direction,
keeping its length.

crates/tools/src/xform/measure.rs

@@ -27,7 +27,8 @@ pub fn measure(a: Point, b: Point) -> (f64, f64, f64, f64) {
 /// it. With Shift held, the end stays on the 45° step from the Constrain Angle nearest the
 /// pointer's direction: `p` is projected onto the step, then snapped along it as the Line Segment
 /// tool snaps a Shift-drawn end. With Snap to Point alone, the point `p` snaps to (an anchor, or
-/// in line with a ruler guide) is projected instead.
+/// in line with a ruler guide) is projected instead. When the snapped point lies nearer another
+/// step (the grid rounds it past the halfway angle), it is turned back onto the pointer's step.
 pub(crate) fn end_point(cx: &ToolContext, a: Point, p: Point, shift: bool) -> Point {
     if !shift {
         return crate::guides::snap_draw(cx, p, &[]).0;
@@ -42,7 +43,9 @@ pub(crate) fn end_point(cx: &ToolContext, a: Point, p: Point, shift: bool) -> Po
     let s = if to_point { crate::guides::snap_draw(cx, p, &[]).0 } else { p };
     let q = a + u * (s - a).dot(u);
     let leave = crate::guides::Leave::segment(cx, a, true);
-    crate::guides::PointSnap::new(cx, || crate::guides::Targets::collect(cx.doc, &[], None)).snap_from(cx, q, Some(&leave)).0
+    let r = crate::guides::PointSnap::new(cx, || crate::guides::Targets::collect(cx.doc, &[], None)).snap_from(cx, q, Some(&leave)).0;
+    let v = r - a;
+    if v.cross(u).abs() <= 1e-9 * v.hypot().max(1.0) { r } else { a + u * v.hypot() }
 }
 
 impl Tool for MeasureTool {
@@ -222,6 +225,27 @@ mod tests {
         assert!((o["dy"].as_f64().unwrap() + 100.0).abs() < 1e-9, "{o}");
     }
 
+    /// With a Constrain Angle of 30°, the grid point a Shift-drag's end rounds to can lie nearer
+    /// another step; the end stays on the step nearest the pointer's direction.
+    #[test]
+    fn shift_drag_keeps_its_step_after_the_grid_rounds_it() {
+        let (d, _) = doc_with_rect();
+        let s = Selection::default();
+        let p = paint();
+        let mut cx = cx(&d, &s, &p);
+        cx.smart_guides = false;
+        cx.snap_to_grid = true;
+        cx.constrain_angle = 30.0;
+        let shift = Mods { shift: true, ..Mods::default() };
+        let mut t = MeasureTool::default();
+        // (5, -3) points 31° up, so the step is 30°; the end lands at about (5.05, -2.92), which
+        // the 9 pt grid rounds to (9, 0), nearer the -15° step.
+        t.pointer(&cx, &PointerEvent::new(PointerKind::Down, 0.0, 0.0));
+        t.pointer(&cx, &PointerEvent::new(PointerKind::Up, 5.0, -3.0).with_mods(shift));
+        let o = t.options();
+        assert!((o["angle"].as_f64().unwrap() - 30.0).abs() < 1e-9, "{o}");
+    }
+
     /// With Snap to Point alone, a Shift-drag's end slides along its 45° step to a ruler guide.
     #[test]
     fn shift_drag_meets_a_ruler_guide() {