Selection: with Real-time Drawing off, the bounding box moves with a move drag

With Preferences › Performance › Real-time Drawing and Editing off, a
Selection tool move drag leaves the object in place until release, and
the bounding box was hidden for the whole drag. The box, with its
handles, now follows the pointer during the drag (Session::deferred_move
gives the offset the document has not taken yet). Scale and rotate
drags, and drags with Real-time Drawing on, hide the box as before.

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

crates/engine/src/tooling.rs

@@ -572,6 +572,24 @@ impl Session {
         self.tool.transforming()
     }
 
+    /// With Real-time Drawing off, how far a Selection-tool move drag has carried the selection
+    /// so far: the document moves on release, and the bounding box shows at this offset meanwhile.
+    pub fn deferred_move(&self) -> Option<vectorcraft_geom::Vec2> {
+        if self.tool.id() != "selection" || !self.tool.transforming() {
+            return None;
+        }
+        let it = self.active()?.interaction.as_ref()?;
+        if !matches!(it.label.as_str(), "Move" | "Copy") {
+            return None;
+        }
+        let (cmd, p) = self.unapplied_interaction_preview()?;
+        if cmd != "object.transform" || p.get("objects").and_then(Value::as_bool) == Some(false) {
+            return None;
+        }
+        let [a, b, c, d, e, f] = crate::cmd::matrix_param(&p, "matrix")?.as_coeffs();
+        (a == 1.0 && b == 0.0 && c == 0.0 && d == 1.0).then(|| vectorcraft_geom::Vec2::new(e, f))
+    }
+
     /// Does the active tool take `key` ahead of the shortcuts bound to it (see `Tool::claims_key`)?
     pub fn tool_claims_key(&mut self, key: ToolKey, view: ViewInfo) -> bool {
         self.plane_key(key).is_some() || self.with_tool_cx(view, |t, cx| t.claims_key(cx, key))