fix(render): environment-aware atlas drift guard, pinned text scale, shipped-atlas coverage

tests/test_render_smoke.cpp

@@ -426,6 +426,61 @@ class CountFenceWaits {
 
 }  // namespace
 
+// The renderer draws a run at the size the style asked for, not at a whole
+// multiple of the backend's cell, and it reports what it drew. `axis_font_px()`
+// is that report, and the committed atlas is baked at 13 px, so the default
+// style's 12 px is a 12/13 scale -- exactly the case the old rule snapped to
+// 16 px. No other case sets a size other than the default, and the ones that
+// compare a measure compare values scaled by the same factor, so they hold under
+// a snapping rule too; this is the one that does not.
+PLOT_TEST(axis_text_is_drawn_at_the_requested_em_not_a_whole_multiple) {
+  plottest::HeadlessContext ctx(kW, kH);
+  plot::Status st;
+  PLOT_CHECK(ctx.create(&st));
+  PLOT_CHECK(plot::gl::load(nullptr));
+
+  // The renderer's own backend, for the raw metrics the ratio is taken against.
+  const plot::render::EmbeddedMonoFont font;
+  const plot::f32 baked = font.em_px();
+  const plot::f32 cell = font.height();
+
+  plot::Renderer r;
+  r.style().axis_font_px = 12.0f;  // the default style's em, against a 13 px bake
+  PLOT_CHECK(r.init(&st));
+  PLOT_CHECK_NEAR(r.axis_font_px(), cell * (12.0f / baked), 1e-3);
+  PLOT_CHECK(std::fabs(r.axis_font_px() - 16.0f) > 0.2f);  // 2 x the old 8 px glyph
+  PLOT_CHECK(std::fabs(r.axis_font_px() - 17.0f) > 0.2f);  // 1 x this cell
+
+  plot::Plot p;
+  p.set_viewport({0.0f, 0.0f, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+  p.add_x_axis({"t", {0.0, 1000.0}, true, plot::AxisPlace::Bottom, false});
+  p.add_y_axis({"v", {-1.0, 1.0}, true, plot::AxisPlace::Left, false});
+  const plot::Rect viewport{0.0f, 0.0f, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)};
+  r.begin_frame(kW, kH, viewport);
+  r.draw(p);
+  r.end_frame();
+
+  // The plot lays out with the backend the renderer installed, so the size it
+  // measured with is the size it drew with.
+  const plot::render::TextBackend* backend = p.text_backend();
+  PLOT_CHECK(backend != nullptr);
+  PLOT_CHECK_NEAR(backend->em_px(), 12.0f, 1e-3);
+  PLOT_CHECK_NEAR(backend->height(), cell * (12.0f / baked), 1e-3);
+  // `em_px()` is in the wrapper's units, like `height()`: a caller drawing at
+  // `requested_px / em_px()` reads 12/12 = 1 and cannot scale a second time.
+  PLOT_CHECK_NEAR(backend->em_px() / backend->height(), baked / cell, 1e-6);
+
+  // An exact multiple still lands exactly, and the frame re-reads the style.
+  r.style().axis_font_px = 26.0f;  // 2 x 13
+  r.begin_frame(kW, kH, viewport);
+  r.draw(p);
+  r.end_frame();
+  PLOT_CHECK_NEAR(p.text_backend()->em_px(), 26.0f, 1e-3);
+  PLOT_CHECK_NEAR(p.text_backend()->height(), cell * 2.0f, 1e-3);
+  PLOT_CHECK(plot::gl::glGetError() == plot::gl::GL_NO_ERROR);
+  ctx.destroy();
+}
+
 PLOT_TEST(renders_visible_trace_across_viewport) {
   plottest::HeadlessContext ctx(kW, kH);
   plot::Status st;