plotter/adapters/gtk4/gtk_plot_area.cpp

#include <gtk4/gtk_plot_area.hpp>

#include <gtk4/gtk_input.hpp>

#include <plot/gl/loader.hpp>

// GTK after this library's GL surface, the order every adapter here keeps. GTK's
// headers pull in no `<GL/gl.h>` -- GdkGLContext is opaque and GtkGLArea names no
// GL type -- so unlike Qt there is no macro to dodge, but the order is the
// project's rule and it keeps the two from ever having to be thought about.
#include <gtk/gtk.h>

#include <chrono>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <dlfcn.h>

// The mirrored constants in gtk_input.hpp are pinned to GDK's here: a GDK that
// renumbered the scroll units or moved a modifier bit fails to compile rather
// than mis-scaling a zoom or dropping a modifier.
static_assert(static_cast<int>(plot::gtk4::ScrollUnit::Wheel) == GDK_SCROLL_UNIT_WHEEL,
              "ScrollUnit::Wheel must match GDK_SCROLL_UNIT_WHEEL");
static_assert(static_cast<int>(plot::gtk4::ScrollUnit::Surface) == GDK_SCROLL_UNIT_SURFACE,
              "ScrollUnit::Surface must match GDK_SCROLL_UNIT_SURFACE");
static_assert((1u << 0) == GDK_SHIFT_MASK, "mods_from's shift bit must match GDK's");
static_assert((1u << 2) == GDK_CONTROL_MASK, "mods_from's control bit must match GDK's");
static_assert((1u << 3) == GDK_ALT_MASK, "mods_from's alt bit must match GDK's");
static_assert((1u << 26) == GDK_SUPER_MASK, "mods_from's super bit must match GDK's");
static_assert((1u << 28) == GDK_META_MASK, "mods_from's meta bit must match GDK's");

namespace plot::gtk4 {
namespace {

/// The entry points the resolver below could not provide, as passed to
/// `plot::gl::load()`. `load()` may still cover them through its own dlsym
/// fallback, but only a context-aware resolver is guaranteed to return pointers
/// valid for the current context, so a fallback is worth reporting. Process-global
/// and written only from the context's owning thread, which is the only thread
/// `::realize` runs on.
constexpr u32 kMaxUnresolved = 16;
const char* g_unresolved[kMaxUnresolved] = {};
u32 g_unresolved_count = 0;

/// `dlsym()` hands back an object pointer for a symbol that is in fact a
/// function. POSIX guarantees the two have the same representation, and the bit
/// copy states that where a `reinterpret_cast` is merely conditionally supported
/// -- the same respelling the GLFW and Qt adapters use for their own resolvers.
template <typename Fn>
Fn symbol_as(void* symbol) noexcept {
  Fn fn = nullptr;
  static_assert(sizeof(fn) == sizeof(symbol), "function and object pointers must match");
  std::memcpy(&fn, &symbol, sizeof(fn));
  return fn;
}

/// The `*GetProcAddress` of the API that owns GTK's current context, as
/// `plot::gl::ProcLoader`.
///
/// GDK4 offers no equivalent: `gdk_gl_context_get_proc_address()` was removed from
/// GTK4's public API, and GTK resolves its own GL entry points through libepoxy.
/// The context a `GtkGLArea` gets comes from EGL on both the X11 and the Wayland
/// backend, so `eglGetProcAddress()` is the context-aware resolver for it -- the
/// call every EGL client makes. GLX is tried second, for a GDK built on it. Both
/// are reached with `dlsym(RTLD_DEFAULT, ...)`, which finds them because libgtk
/// links libEGL and libGLX; neither is called when it is not there, in which case
/// the name is reported and `load()`'s own fallback gets its chance.
void* resolve_gl_proc(const char* name) noexcept {
  using GetProcAddress = void* (*)(const char*);
  static const GetProcAddress egl =
      symbol_as<GetProcAddress>(dlsym(RTLD_DEFAULT, "eglGetProcAddress"));
  static const GetProcAddress glx =
      symbol_as<GetProcAddress>(dlsym(RTLD_DEFAULT, "glXGetProcAddressARB"));

  void* proc = nullptr;
  if (egl != nullptr) proc = egl(name);
  if (proc == nullptr && glx != nullptr) proc = glx(name);
  if (proc == nullptr && g_unresolved_count < kMaxUnresolved) {
    g_unresolved[g_unresolved_count++] = name;
  }
  return proc;
}

/// Monotonic seconds, for the frame timings only.
f64 now_s() noexcept {
  return std::chrono::duration<f64>(std::chrono::steady_clock::now().time_since_epoch()).count();
}

/// `gtk_event_controller_get_current_event_state()` as the controller's mask.
u32 gesture_mods(GtkEventController* controller) noexcept {
  return mods_from(static_cast<u32>(gtk_event_controller_get_current_event_state(controller)));
}

/// The position of the event a controller is handling right now, in device
/// pixels. False when there is no event or it has no position -- a key event with
/// a scroll controller's anchor is the case that matters.
bool current_event_position(GtkEventController* controller, f32 scale, Vec2* out) noexcept {
  GdkEvent* event = gtk_event_controller_get_current_event(controller);
  if (event == nullptr) return false;
  double x = 0.0;
  double y = 0.0;
  if (!gdk_event_get_position(event, &x, &y)) return false;
  *out = Vec2{static_cast<f32>(x) * scale, static_cast<f32>(y) * scale};
  return true;
}

/// The widget's allocation in device pixels: what the plot is given, what GTK
/// attaches as the framebuffer, and what `GtkGLArea::resize` reports. Zero-sized
/// before the widget is allocated.
Rect alloc_in_device(GtkWidget* widget) noexcept {
  const f32 scale = static_cast<f32>(gtk_widget_get_scale_factor(widget));
  return Rect{0.0f, 0.0f, static_cast<f32>(gtk_widget_get_width(widget)) * scale,
              static_cast<f32>(gtk_widget_get_height(widget)) * scale};
}

}  // namespace

PlotArea* PlotArea::create(Plot& plot) noexcept {
  GtkWidget* widget = gtk_gl_area_new();

  // The context is asked for at construction: GTK reads these when it creates the
  // context at realize, and `gtk_gl_area_set_required_version` is documented to
  // have no effect afterwards. 3.3 core is the baseline the library's loader
  // resolves; the plot is 2D, so a depth buffer and a stencil buffer would only
  // cost memory (GTK's own default is none, stated explicitly so a GTK that
  // changed it could not silently add one).
  gtk_gl_area_set_required_version(GTK_GL_AREA(widget), 3, 3);
  gtk_gl_area_set_has_depth_buffer(GTK_GL_AREA(widget), FALSE);
  gtk_gl_area_set_has_stencil_buffer(GTK_GL_AREA(widget), FALSE);

  // A widget that cannot take focus never receives a key event, and the
  // controller's `+`, `-` and `f` are key events.
  gtk_widget_set_focusable(widget, TRUE);
  gtk_widget_set_hexpand(widget, TRUE);
  gtk_widget_set_vexpand(widget, TRUE);

  auto* self = new PlotArea(plot, widget);
  // The wrapper dies with the widget. `::unrealize` has already released the GL
  // objects by then, while the context could still be made current.
  g_object_set_data_full(G_OBJECT(widget), "plot-gtk4-area", self,
                         [](gpointer data) { delete static_cast<PlotArea*>(data); });

  g_signal_connect(widget, "realize", G_CALLBACK(&PlotArea::on_realize), self);
  g_signal_connect(widget, "unrealize", G_CALLBACK(&PlotArea::on_unrealize), self);
  g_signal_connect(widget, "render", G_CALLBACK(&PlotArea::on_render), self);
  g_signal_connect(widget, "resize", G_CALLBACK(&PlotArea::on_resize), self);

  // One click gesture and one drag gesture, both listening to every button: the
  // plot pans on left *and* right, box-zooms on shift+left, moves a cursor on
  // ctrl+left and zooms an axis on middle. GTK cannot be asked for that set
  // directly, so the two gestures split it (see the class comment) and the button
  // and modifiers decide which gesture the controller runs.
  GtkGesture* click = gtk_gesture_click_new();
  gtk_gesture_single_set_button(GTK_GESTURE_SINGLE(click), 0);  // 0: any button
  g_signal_connect(click, "pressed", G_CALLBACK(&PlotArea::on_click_pressed), self);
  g_signal_connect(click, "released", G_CALLBACK(&PlotArea::on_click_released), self);
  g_signal_connect(click, "stopped", G_CALLBACK(&PlotArea::on_click_stopped), self);
  gtk_widget_add_controller(widget, GTK_EVENT_CONTROLLER(click));

  GtkGesture* drag = gtk_gesture_drag_new();
  gtk_gesture_single_set_button(GTK_GESTURE_SINGLE(drag), 0);
  g_signal_connect(drag, "drag-begin", G_CALLBACK(&PlotArea::on_drag_begin), self);
  g_signal_connect(drag, "drag-update", G_CALLBACK(&PlotArea::on_drag_update), self);
  g_signal_connect(drag, "drag-end", G_CALLBACK(&PlotArea::on_drag_end), self);
  gtk_widget_add_controller(widget, GTK_EVENT_CONTROLLER(drag));

  // Vertical only, as the plan asks: a purely horizontal scroll is not a zoom, and
  // because this controller never sees one it cannot be mistaken for one. GTK
  // reports the unit per event, so a discrete wheel arrives in notches and a
  // smooth device in surface pixels; both are converted before they reach the
  // controller (see scroll_notches).
  GtkEventController* scroll =
      gtk_event_controller_scroll_new(GTK_EVENT_CONTROLLER_SCROLL_VERTICAL);
  g_signal_connect(scroll, "scroll", G_CALLBACK(&PlotArea::on_scroll), self);
  gtk_widget_add_controller(widget, scroll);

  // Hover: the controller's cursor hit test reads the last pointer position even
  // while no button is down.
  GtkEventController* motion = gtk_event_controller_motion_new();
  g_signal_connect(motion, "motion", G_CALLBACK(&PlotArea::on_motion), self);
  gtk_widget_add_controller(widget, motion);

  GtkEventController* key = gtk_event_controller_key_new();
  g_signal_connect(key, "key-pressed", G_CALLBACK(&PlotArea::on_key_pressed), self);
  gtk_widget_add_controller(widget, key);

  return self;
}

PlotArea::PlotArea(Plot& plot, GtkWidget* widget) noexcept
    : plot_(&plot), controller_(plot), widget_(widget) {}

Rect PlotArea::device_viewport() const noexcept { return alloc_in_device(widget_); }

int PlotArea::scale_factor() const noexcept { return gtk_widget_get_scale_factor(widget_); }

Vec2 PlotArea::to_device(double x, double y) const noexcept {
  // The factor `device_viewport()` scales the widget by, applied to a position and
  // to the offsets derived from two of them: one convention on both sides is what
  // makes a gesture land on the data under the cursor.
  const f32 scale = static_cast<f32>(gtk_widget_get_scale_factor(widget_));
  return Vec2{static_cast<f32>(x) * scale, static_cast<f32>(y) * scale};
}

void PlotArea::refresh_viewport() noexcept {
  // The controller owns no viewport: this hands it to the plot (which re-lays the
  // chrome out with it) and refreshes the layout. `last_device_viewport()` is
  // deliberately not assigned here -- it records the region a *frame* was drawn
  // into, which is `render()`'s to set.
  controller_.set_viewport(device_viewport());
}

// --- the GL signals -------------------------------------------------------

void PlotArea::realize() noexcept {
  // A second realize means GTK created a new context for a widget that moved to
  // another surface: the names the renderer holds belonged to the old one, and
  // nothing here may outlive it. Released before anything of ours exists on the
  // new context, so the deletes it issues can only name objects that are not
  // there.
  renderer_.reset();

  gtk_gl_area_make_current(GTK_GL_AREA(widget_));
  GError* error = gtk_gl_area_get_error(GTK_GL_AREA(widget_));
  if (error != nullptr) {
    // The context GTK was asked for does not exist: no GL entry point may be
    // called, and the area draws nothing. Reported, not asserted -- a GTK signal
    // handler has nowhere to return a failure to.
    init_status_ = Status{ErrorCode::GLFailure,
                          "GtkGLArea could not create a GL 3.3 context; see the GTK warning above"};
    std::fprintf(stderr, "[plot::gtk4] GLFailure: %s (%s)\n", init_status_.message,
                 error->message != nullptr ? error->message : "no detail");
    return;
  }

  g_unresolved_count = 0;
  const bool loaded = plot::gl::load(&resolve_gl_proc);
  // Printed before the failure branch: the names collected here are what explains
  // a failure, so a load that failed has to report them too.
  if (g_unresolved_count > 0) {
    std::fprintf(stderr,
                 "[plot::gtk4] eglGetProcAddress() did not resolve %u required entry point(s);"
                 " plot::gl::load() %s for:",
                 g_unresolved_count, loaded ? "used its dlsym fallback" : "failed");
    for (u32 i = 0; i < g_unresolved_count; ++i) std::fprintf(stderr, " %s", g_unresolved[i]);
    std::fprintf(stderr, "\n");
  }
  if (!loaded) {
    init_status_ = Status{ErrorCode::Unsupported,
                          "plot::gl::load() failed: the current context is missing a required"
                          " OpenGL 3.3 entry point"};
    std::fprintf(stderr, "[plot::gtk4] %s\n", init_status_.message);
    return;
  }

  renderer_.emplace();
  Status status;
  if (!renderer_->init(&status)) {
    init_status_ = status.code != ErrorCode::Ok ? status
                                                : Status{ErrorCode::GLFailure,
                                                         "Renderer::init() failed"};
    // The renderer stays engaged: its destructor releases whatever the partial
    // init built, and it runs in unrealize() with the context current.
    std::fprintf(stderr, "[plot::gtk4] Renderer::init(): %s: %s\n", to_string(init_status_.code),
                 init_status_.message);
    return;
  }
  init_status_ = Status{};
  refresh_viewport();
}

void PlotArea::unrealize() noexcept {
  // The context is still alive here and can still be made current (GTK clears it
  // after this signal), which is what the renderer's RAII handles need: a
  // `glDelete*` against a context that is going away -- or against no context at
  // all -- is worse than the leak it would avoid.
  if (renderer_) {
    gtk_gl_area_make_current(GTK_GL_AREA(widget_));
    if (gtk_gl_area_get_error(GTK_GL_AREA(widget_)) == nullptr) renderer_.reset();
  }
  press_open_ = false;
  drag_active_ = false;
}

void PlotArea::render() noexcept {
  // GTK guarantees the context is current and the area's FBO is bound when
  // `::render` runs; making it current is a no-op in that case, and the error
  // check is the assertion that it holds rather than an assumption. Both are
  // `gtk_gl_area_make_current()`'s documented behaviour, and the check costs one
  // call per frame -- the alternative is drawing through entry points that
  // belonged to a context GTK has already thrown away.
  gtk_gl_area_make_current(GTK_GL_AREA(widget_));
  if (gtk_gl_area_get_error(GTK_GL_AREA(widget_)) != nullptr) return;
  if (!renderer_ || !renderer_->ready()) return;

  const Rect viewport = device_viewport();
  if (viewport.w < 1.0f || viewport.h < 1.0f) return;  // nothing to draw into

  // The controller owns no viewport: this hands it to the plot (which re-lays the
  // chrome out with it) and refreshes the layout, so every frame is laid out for
  // the geometry of *this* frame. A resize changes the layout; it never scales a
  // stale one.
  controller_.set_viewport(viewport);

  const f64 started = now_s();
  const f64 dt = last_render_s_ >= 0.0 ? started - last_render_s_ : 0.0;
  last_render_s_ = started;
  if (frame_) frame_(*plot_, dt);

  renderer_->begin_frame(static_cast<u32>(viewport.w), static_cast<u32>(viewport.h), viewport);
  renderer_->draw(*plot_);
  renderer_->end_frame();

  const f32 frame_ms = static_cast<f32>((now_s() - started) * 1.0e3);
  last_frame_ms_ = frame_ms;
  // Seeded by the first frame rather than started at zero, so the mean is the mean
  // of the frames that ran from the first one.
  mean_frame_ms_ = frames_ == 0 ? frame_ms : mean_frame_ms_ + 0.1f * (frame_ms - mean_frame_ms_);
  last_viewport_ = viewport;
  ++frames_;
}

void PlotArea::on_realize(GtkGLArea*, void* self) noexcept {
  static_cast<PlotArea*>(self)->realize();
}

void PlotArea::on_unrealize(GtkGLArea*, void* self) noexcept {
  static_cast<PlotArea*>(self)->unrealize();
}

int PlotArea::on_render(GtkGLArea*, GdkGLContext*, void* self) noexcept {
  static_cast<PlotArea*>(self)->render();
  // FALSE on purpose, not "I did not draw": `GtkGLArea::render`'s return value is
  // accumulated with `g_signal_accumulator_true_handled`, so the first handler to
  // return TRUE *stops the emission* -- and a host that connects its own handler
  // (a screenshot read, a frame counter) would never be called. GTK itself does
  // not read the value (it composites the FBO the area rendered into either way),
  // and a GtkGLArea subclass overriding the vfunc is the only place TRUE means
  // anything.
  return FALSE;
}

void PlotArea::on_resize(GtkGLArea*, int width, int height, void* self) noexcept {
  (void)width;
  (void)height;
  // Nothing about the size is cached from these two, on purpose: device_viewport()
  // re-derives it from the live allocation, which is what the input mapping reads
  // as well. GTK's numbers are the same ones -- it reports the GL viewport in
  // device pixels, and this area's `width * scale` is that -- but one derivation
  // is what keeps a gesture, a frame and a screenshot from ever disagreeing.
  // Refreshing it here means a gesture arriving between the resize and the next
  // frame maps against the new geometry rather than the old one.
  static_cast<PlotArea*>(self)->refresh_viewport();
}

// --- pointer input --------------------------------------------------------

void PlotArea::pointer_press(input::Button button, u32 mods, double x, double y) noexcept {
  if (press_open_) return;  // one press per button-down sequence: see drag_begin
  press_open_ = true;
  press_button_ = button;
  press_mods_ = mods;

  const Vec2 p = to_device(x, y);
  pointer_ = p;
  have_pointer_ = true;

  input::PointerEvent e;
  e.x = p.x;
  e.y = p.y;
  e.dx = 0.0f;
  e.dy = 0.0f;
  e.button = button;
  e.mods = mods;
  e.down = true;
  e.pressed = true;
  controller_.on_pointer(e);
  gtk_gl_area_queue_render(GTK_GL_AREA(widget_));
}

void PlotArea::pointer_release(Vec2 position) noexcept {
  if (!press_open_) return;
  press_open_ = false;

  input::PointerEvent e;
  e.x = position.x;
  e.y = position.y;
  e.dx = 0.0f;
  e.dy = 0.0f;
  e.button = press_button_;
  e.mods = press_mods_;
  e.down = false;
  e.released = true;
  controller_.on_pointer(e);
  gtk_gl_area_queue_render(GTK_GL_AREA(widget_));
}

void PlotArea::on_click_pressed(GtkGestureClick* gesture, int, double x, double y,
                                void* self) noexcept {
  auto* area = static_cast<PlotArea*>(self);
  const std::optional<input::Button> button =
      button_from(gtk_gesture_single_get_current_button(GTK_GESTURE_SINGLE(gesture)));
  if (!button) return;  // a button the controller has no gesture for
  // The modifiers come off the *event*, not the gesture: GTK reports the state as
  // it was before the press, which is what the controller's gesture table reads.
  area->pointer_press(*button, gesture_mods(GTK_EVENT_CONTROLLER(gesture)), x, y);
}

void PlotArea::on_click_released(GtkGestureClick* gesture, int, double x, double y,
                                 void* self) noexcept {
  (void)gesture;
  auto* area = static_cast<PlotArea*>(self);
  // The first release closes the gesture; a drag's own `drag-end` may already have
  // done it (GTK emits both, `drag-end` first), and this is then a no-op.
  area->pointer_release(area->to_device(x, y));
}

void PlotArea::on_click_stopped(GtkGestureClick* gesture, void* self) noexcept {
  (void)gesture;
  auto* area = static_cast<PlotArea*>(self);
  // A sequence can be cancelled rather than released -- GTK stops the click
  // gesture as soon as the drag gesture claims the same sequence, which happens on
  // the first motion of every drag. That stop must not end the gesture: the drag
  // is still running, and its updates keep driving the plot. Only a stop with no
  // drag in progress (an exotic cancellation by another widget or a device) closes
  // the press, which it has to, or the controller would stay in whatever mode the
  // press began.
  if (area->press_open_ && !area->drag_active_) area->pointer_release(area->pointer_);
}

void PlotArea::on_drag_begin(GtkGestureDrag* gesture, double start_x, double start_y,
                             void* self) noexcept {
  auto* area = static_cast<PlotArea*>(self);
  area->drag_active_ = true;
  area->drag_start_ = area->to_device(start_x, start_y);
  area->drag_offset_ = Vec2{0.0f, 0.0f};
  area->pointer_ = area->drag_start_;
  area->have_pointer_ = true;

  // GTK emits `drag-begin` before the click gesture's `pressed` (both while the
  // press event is handled), so this is normally the second press of the sequence
  // and `pointer_press` drops it. It is not always: if the click gesture never
  // pressed -- a host that grabs the sequence, a GTK that reorders its gestures --
  // the press is opened here instead, so a drag cannot be lost for want of an
  // edge. The button and modifiers come from the drag gesture for that case.
  const std::optional<input::Button> button =
      button_from(gtk_gesture_single_get_current_button(GTK_GESTURE_SINGLE(gesture)));
  if (button) {
    area->pointer_press(*button, gesture_mods(GTK_EVENT_CONTROLLER(gesture)), start_x, start_y);
  }
}

void PlotArea::on_drag_update(GtkGestureDrag* gesture, double offset_x, double offset_y,
                              void* self) noexcept {
  (void)gesture;
  auto* area = static_cast<PlotArea*>(self);
  // A drag with no press open is a drag of a button the adapter does not forward
  // (see button_from): its motion must not reach the controller as a held-drag
  // move, which is the one event the controller acts on from its button state.
  if (!area->press_open_) return;
  const Vec2 offset = area->to_device(offset_x, offset_y);
  // The drag gesture reports the offset from the press point, the controller wants
  // the movement since the previous sample: the difference of two offsets, both
  // already scaled by the same factor as the viewport and the press.
  const Vec2 delta = Vec2{offset.x - area->drag_offset_.x, offset.y - area->drag_offset_.y};
  area->drag_offset_ = offset;
  const Vec2 position = Vec2{area->drag_start_.x + offset.x, area->drag_start_.y + offset.y};
  area->pointer_ = position;
  area->have_pointer_ = true;

  input::PointerEvent e;
  e.x = position.x;
  e.y = position.y;
  e.dx = delta.x;
  e.dy = delta.y;
  e.button = area->press_button_;
  e.mods = area->press_mods_;
  e.down = true;
  area->controller_.on_pointer(e);
  gtk_gl_area_queue_render(GTK_GL_AREA(area->widget_));
}

void PlotArea::on_drag_end(GtkGestureDrag* gesture, double offset_x, double offset_y,
                           void* self) noexcept {
  (void)gesture;
  auto* area = static_cast<PlotArea*>(self);
  area->drag_active_ = false;
  // The release carries the authoritative end of the gesture: the controller reads
  // its position to finish a box zoom's selection, so this is the press point plus
  // the final offset and not the last update's.
  const Vec2 offset = area->to_device(offset_x, offset_y);
  area->pointer_ = Vec2{area->drag_start_.x + offset.x, area->drag_start_.y + offset.y};
  area->pointer_release(area->pointer_);
}

int PlotArea::on_scroll(GtkEventControllerScroll* scroll, double dx, double dy,
                        void* self) noexcept {
  auto* area = static_cast<PlotArea*>(self);
  const ScrollUnit unit = gtk_event_controller_scroll_get_unit(scroll) == GDK_SCROLL_UNIT_SURFACE
                              ? ScrollUnit::Surface
                              : ScrollUnit::Wheel;
  const f64 notches = scroll_notches(dy, unit);
  if (notches == 0.0) {
    // Not a zoom, so not this widget's event: returning FALSE lets an enclosing
    // scrolled window scroll instead. A zero-notch zoom is not harmless -- it
    // changes no range but still runs the range setters, which clear auto scale --
    // so it must not reach the controller either.
    return FALSE;
  }

  const f32 scale = static_cast<f32>(gtk_widget_get_scale_factor(area->widget_));
  Vec2 at{};
  if (!current_event_position(GTK_EVENT_CONTROLLER(scroll), scale, &at)) {
    if (!area->have_pointer_) return FALSE;  // no anchor to zoom about
    at = area->pointer_;  // the last position a pointer event reported
  }
  area->pointer_ = at;
  area->have_pointer_ = true;

  input::WheelEvent e;
  e.x = at.x;
  e.y = at.y;
  e.dx = static_cast<f32>(scroll_notches(dx, unit));
  e.dy = static_cast<f32>(notches);
  e.mods = gesture_mods(GTK_EVENT_CONTROLLER(scroll));
  area->controller_.on_wheel(e);
  gtk_gl_area_queue_render(GTK_GL_AREA(area->widget_));
  return TRUE;  // consumed: the plot zoomed
}

void PlotArea::on_motion(GtkEventControllerMotion* motion, double x, double y, void* self) noexcept {
  auto* area = static_cast<PlotArea*>(self);
  const Vec2 p = area->to_device(x, y);
  area->pointer_ = p;
  area->have_pointer_ = true;

  // Hover only, with `down` false whatever the buttons are doing: a motion event
  // that claimed a button was held would be fed to `update_drag` as well, and a
  // drag would then be applied twice -- once from the drag gesture's offset and
  // once from this position's difference.
  (void)motion;
  input::PointerEvent e;
  e.x = p.x;
  e.y = p.y;
  e.dx = 0.0f;
  e.dy = 0.0f;
  e.down = false;
  area->controller_.on_pointer(e);
}

// --- keyboard -------------------------------------------------------------

int PlotArea::on_key_pressed(GtkEventControllerKey* keys, unsigned int keyval,
                             unsigned int keycode, unsigned int state, void* self) noexcept {
  (void)keycode;
  (void)keys;
  auto* area = static_cast<PlotArea*>(self);
  // The keys are named by their *symbol*: GTK reports a layout's keyval, and the
  // keypad's plus and minus are the same symbol as the main row's. `'='` is mapped
  // with `'+'` so both spellings of "zoom in" work (on most layouts `+` *is*
  // shift+`=`), the pair the GLFW and Qt adapters map.
  int key = 0;
  switch (gdk_keyval_to_unicode(keyval)) {
    case static_cast<guint32>(input::kKeyZoomIn):
    case static_cast<guint32>('='): key = input::kKeyZoomIn; break;
    case static_cast<guint32>(input::kKeyZoomOut):
    case static_cast<guint32>('_'): key = input::kKeyZoomOut; break;
    case static_cast<guint32>(input::kKeyFit):
    case static_cast<guint32>(input::kKeyFitUpper): key = input::kKeyFit; break;
    default: return FALSE;  // not a plot key: GTK and the window keep it
  }

  input::KeyEvent e;
  e.key = key;
  e.mods = mods_from(state);
  // GTK reports a held key as repeated presses and sends no release the controller
  // would act on anyway (its actions are press-only), so the press is the event.
  e.down = true;
  area->controller_.on_key(e);
  gtk_gl_area_queue_render(GTK_GL_AREA(area->widget_));
  return TRUE;  // consumed: the plot acted on it
}

}  // namespace plot::gtk4