feat(input): draggable cursors, regions and measurement-band edges

tests/test_controller.cpp

@@ -1093,3 +1093,65 @@ PLOT_TEST(band_edge_drag_moves_the_owning_cursor_and_updates_the_measurement) {
   PLOT_CHECK_NEAR(narrowed.ymax, 50.0, 1e-9);
   release(f.controller, to_b);
 }
+
+/// A region is geometry in *data* coordinates, and `XFollow::Live` recomputes the x window
+/// on every layout -- so the two claims that matter under a live axis are that the region
+/// stays exactly where it was put in data space while the view scrolls under it, and that
+/// dragging one moves the region rather than being read as a change to the range. The
+/// live path owns the range, so both halves are things it could plausibly break: a region
+/// measured in pixels would drift with the window, and a drag that reached the range
+/// setters would move the view *and* clear nothing good.
+PLOT_TEST(region_holds_in_data_space_under_live_follow) {
+  LiveFixture f(40, 200);  // a saturated ring: the window slides, its width does not
+  f.live();
+  f.graph.layout();
+  const f64 window_span = f.xr().span();
+  const f64 x0 = f.xr().min + 0.25 * window_span;
+  const f64 x1 = f.xr().min + 0.65 * window_span;
+  const u32 rid = f.graph.add_region(free_region(x0, x1));
+  PLOT_CHECK_NEAR(f.graph.region(rid).x0, x0, 1e-12);
+  PLOT_CHECK_NEAR(f.graph.region(rid).x1, x1, 1e-12);
+  const f32 left_before = bound_x(f.graph, rid, true);
+  const f32 right_before = bound_x(f.graph, rid, false);
+  PLOT_CHECK(left_before >= 0.0f && right_before > left_before);
+
+  // The window advances on its own, which is what the follow path is for.
+  const f64 newest_before = f.newest();
+  f.feed(3);
+  f.graph.layout();
+  PLOT_CHECK(f.xr().max > newest_before);
+  PLOT_CHECK(f.graph.x_follow(f.ax) == plot::XFollow::Live);
+  // The region's own interval is untouched, to the last bit ...
+  PLOT_CHECK_NEAR(f.graph.region(rid).x0, x0, 1e-12);
+  PLOT_CHECK_NEAR(f.graph.region(rid).x1, x1, 1e-12);
+  // ... while the pixels it is drawn in moved: the view scrolled under a region that
+  // stayed in data space.
+  PLOT_CHECK(bound_x(f.graph, rid, true) < left_before);
+  PLOT_CHECK(bound_x(f.graph, rid, false) < right_before);
+
+  // A body drag moves the region and nothing else: not the window, not the follow state.
+  const plot::Range held = f.xr();
+  const f32 mid = 0.5f * (bound_x(f.graph, rid, true) + bound_x(f.graph, rid, false));
+  const f32 to = mid + 40.0f;
+  const f64 delta = data_at(f.graph, f.ax, to) - data_at(f.graph, f.ax, mid);
+  press(f.controller, mid);
+  PLOT_CHECK(f.controller.drag_mode() == plot::input::DragMode::MoveRegion);
+  PLOT_CHECK(f.controller.grabbed_region() == rid);
+  move_to(f.controller, to);
+  PLOT_CHECK_NEAR(f.graph.region(rid).x0, x0 + delta, 1e-9);
+  PLOT_CHECK_NEAR(f.graph.region(rid).x1, x1 + delta, 1e-9);
+  PLOT_CHECK_NEAR(f.xr().min, held.min, 1e-9);  // the range the follow path owns is untouched
+  PLOT_CHECK_NEAR(f.xr().max, held.max, 1e-9);
+  PLOT_CHECK(f.graph.x_follow(f.ax) == plot::XFollow::Live);
+  release(f.controller, to);
+  PLOT_CHECK(f.graph.x_follow(f.ax) == plot::XFollow::Live);
+  PLOT_CHECK_NEAR(f.graph.region(rid).x0, x0 + delta, 1e-9);
+  PLOT_CHECK_NEAR(f.graph.region(rid).x1, x1 + delta, 1e-9);
+  // And the region is still where the pointer left it once more samples arrive: the
+  // window moves, the region does not.
+  f.feed(2);
+  f.graph.layout();
+  PLOT_CHECK(f.xr().max > held.max);
+  PLOT_CHECK_NEAR(f.graph.region(rid).x0, x0 + delta, 1e-12);
+  PLOT_CHECK_NEAR(f.graph.region(rid).x1, x1 + delta, 1e-12);
+}