fix(plot): a degenerate range cannot strand a live axis, and the demo's follow check fails the run

tests/test_controller.cpp

@@ -681,8 +681,8 @@ PLOT_TEST(drag_zoom_is_logical_the_wheel_is_not_scaled) {
     const f32 drag_px = 20.0f * scale;  // 20 *logical* px at both scales
     c.on_pointer({strip_x, strip_y, 0, 0, plot::input::Button::Middle, 0, true, true, false});
     PLOT_CHECK(c.drag_mode() == plot::input::DragMode::AxisZoomX);
-    c.on_pointer({strip_x, strip_y + drag_px, 0, drag_px, plot::input::Button::Middle, 0, true, false,
-                  false});
+    c.on_pointer({strip_x, strip_y + drag_px, 0, drag_px, plot::input::Button::Middle, 0, true,
+                  false, false});
     c.on_pointer({strip_x, strip_y + drag_px, 0, 0, plot::input::Button::Middle, 0, false, false,
                   true});
     PLOT_CHECK_NEAR(p.x_range(ax).span(), 100.0 * std::pow(10.0, -20.0 / 200.0), 1e-6);
@@ -692,3 +692,32 @@ PLOT_TEST(drag_zoom_is_logical_the_wheel_is_not_scaled) {
     PLOT_CHECK_NEAR(p.x_range(ax).span(), span_before * std::exp(-0.15), 1e-9);
   }
 }
+
+/// The consequences of a degenerate range on a live axis, from the gesture side: the
+/// wheel, the pan and the axis zoom all refuse a span that is not a span, so the
+/// frozen axis a stored zero-span left behind was one no interaction could move --
+/// while `x_follow()` said `Live`. With the guard in `set_x_range` the slips a caller
+/// can actually make leave the gestures working.
+PLOT_TEST(live_gestures_survive_a_degenerate_range) {
+  LiveFixture f(200, 40);
+  f.live();
+  const plot::Rect axes = std::as_const(f.graph).layout().axes;
+  const f64 span = f.xr().span();
+  const f64 newest = f.newest();
+  const plot::input::WheelEvent wheel = strip_wheel(f.graph, 1.0f);
+
+  f.graph.set_x_range(f.ax, {newest, newest});  // the programmatic slip
+  PLOT_CHECK_NEAR(f.xr().span(), span, 1e-9);
+  f.graph.set_x_range(f.ax, {newest - 5.0, newest - 15.0});  // and an inverted one
+  PLOT_CHECK_NEAR(f.xr().span(), 10.0, 1e-9);                // normalised, not stored inverted
+
+  f.controller.on_wheel(wheel);
+  PLOT_CHECK_NEAR(f.xr().span(), 10.0 * std::exp(-0.15), 1e-9);  // the wheel still zooms
+  PLOT_CHECK(f.graph.x_follow(f.ax) == plot::XFollow::Live);
+  const f64 span_after_wheel = f.xr().span();
+  drag(f.controller, axes, 60.0f);  // a left drag: the pan gesture
+  PLOT_CHECK(f.graph.x_offset(f.ax) > 0.0);  // the pan still pans
+  PLOT_CHECK_NEAR(f.xr().span(), span_after_wheel, 1e-9);
+  PLOT_CHECK_NEAR(f.xr().max, f.newest() - f.graph.x_offset(f.ax), 1e-9);
+  PLOT_CHECK(f.graph.x_follow(f.ax) == plot::XFollow::Live);
+}