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

examples/glfw_demo.cpp

@@ -20,7 +20,9 @@
 //    (`XFollow::Live`), driving three wheel notches and a pan through the host's own
 //    controller and printing, at each step, the range's width, its distance behind
 //    the newest sample and the follow state -- the interaction spec §12 is about,
-//    measured on a real stream rather than in a unit test.
+//    measured on a real stream rather than in a unit test. It is a *check*, not a
+//    readout: the run exits non-zero if a measured property fails to hold, or if the
+//    sequence did not get far enough to measure all four.
 //
 // The producer is joined before the stream it writes into is destroyed, and
 // before the host closes the GL context (main's destruction order does that).
@@ -183,10 +185,17 @@ struct Meter {
 /// Each measurement prints the range's width, how far its right edge sits behind the
 /// newest sample, and whether following is still armed. Only the zoom may change the
 /// width, only the pan may change the distance behind, and following must be armed
-/// throughout; the last line of the run says whether that held, so the run is a check
-/// rather than a printout to be read.
+/// throughout. The verdict is carried out of the run in `failed`, so `main()` exits
+/// non-zero on a failed check -- a check whose result only stdout knows is not a check
+/// -- and also when the sequence never completed, since a run that measured nothing
+/// has not passed.
 struct FollowScript {
   bool enabled = false;
+  /// Set by report(); read by main(). False until the checks have all held.
+  bool failed = false;
+  /// Set by report(): the sequence reached a verdict at all. A run that ended before
+  /// the fourth measurement has not passed, it just has not finished.
+  bool completed = false;
   plot::u32 step = 0;
   plot::f64 width[4]{};
   plot::f64 behind[4]{};
@@ -250,7 +259,8 @@ struct FollowScript {
     const plot::Rect axes = p.layout().axes;
     const f32 x = axes.x + 0.5f * axes.w;
     const f32 y = axes.y + 0.5f * axes.h;
-    host.controller().on_pointer({x, y, 0.0f, 0.0f, plot::input::Button::Left, 0, true, true, false});
+    host.controller().on_pointer(
+        {x, y, 0.0f, 0.0f, plot::input::Button::Left, 0, true, true, false});
     host.controller().on_pointer(
         {x + 100.0f, y, 100.0f, 0.0f, plot::input::Button::Left, 0, true, false, false});
     host.controller().on_pointer(
@@ -282,6 +292,8 @@ struct FollowScript {
     const bool pan_only_behind =
         at_edge(behind[0]) && at_edge(behind[1]) && behind[2] > 1e-6 && same(behind[2], behind[3]);
     const bool following = live[0] && live[1] && live[2] && live[3];
+    failed = !(zoom_only_width && pan_only_behind && following);
+    completed = true;
     std::printf(
         "[glfw_demo] follow checks: width changed only by the zoom: %s; distance behind only"
         " by the pan: %s; following armed throughout: %s\n",
@@ -404,6 +416,25 @@ int main() {
     return 1;
   }
 
+  // The live-follow script's verdict, before the summary: a run that asked for the
+  // verification and did not get a passing one fails, whether a check said NO or the
+  // sequence never got far enough to say anything (a deadline too short to reach the
+  // fourth measurement -- a run that measured nothing has not passed).
+  if (script.enabled) {
+    if (script.failed) {
+      std::fprintf(stderr,
+                   "plot_glfw_demo: the live-follow checks failed (see the 'follow checks'"
+                   " line above)\n");
+      return 1;
+    }
+    if (!script.completed) {
+      std::fprintf(stderr,
+                   "plot_glfw_demo: the live-follow script did not complete (raise"
+                   " PLOT_DEMO_EXIT_AFTER_MS)\n");
+      return 1;
+    }
+  }
+
   const plot::Renderer* r = host.renderer();
   const plot::FrameStats stats = r != nullptr ? r->stats() : plot::FrameStats{};
   const plot::Rect axes = std::as_const(p).layout().axes;