plotter/adapters/gtk4/gtk_plot_area.cpp

// SPDX-License-Identifier: MIT
// SPDX-FileCopyrightText: 2026 Martino Ferrari

#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");
static_assert(plot::gtk4::kGdkKeyEscape == GDK_KEY_Escape, "the mirrored Escape keyval must match GDK's");
static_assert(plot::gtk4::kGdkKeyLeft == GDK_KEY_Left, "the mirrored Left keyval must match GDK's");
static_assert(plot::gtk4::kGdkKeyUp == GDK_KEY_Up, "the mirrored Up keyval must match GDK's");
static_assert(plot::gtk4::kGdkKeyRight == GDK_KEY_Right, "the mirrored Right keyval must match GDK's");
static_assert(plot::gtk4::kGdkKeyDown == GDK_KEY_Down, "the mirrored Down keyval 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
/// at `mapping`. 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, const DeviceMapping& mapping,
                            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 = mapping.to_device(x, y);
  return true;
}
}  // 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));

  // The two-finger pinch: `scale-changed` carries the cumulative zoom since the
  // gesture began, and the gesture's bounding-box centre is the focal point -- the
  // pair `ZoomAnchor` folds into the incremental `GestureEvent` the controller
  // takes (zoom about the focal point, focal movement as the pan). No button:
  // unlike the click/drag pair this gesture is touch-only by nature, and a mouse
  // never produces one.
  GtkGesture* zoom = gtk_gesture_zoom_new();
  g_signal_connect(zoom, "begin", G_CALLBACK(&PlotArea::on_zoom_begin), self);
  g_signal_connect(zoom, "scale-changed", G_CALLBACK(&PlotArea::on_zoom_scale_changed), self);
  g_signal_connect(zoom, "end", G_CALLBACK(&PlotArea::on_zoom_end), self);
  gtk_widget_add_controller(widget, GTK_EVENT_CONTROLLER(zoom));

  // 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 mapping_.viewport; }

f32 PlotArea::device_scale() const noexcept { return mapping_.factor; }

Vec2 PlotArea::to_device(double x, double y) const noexcept {
  // The mapping `device_viewport()` is the viewport of, applied to a position and
  // to the offsets derived from two of them: one mapping on both sides is what
  // makes a gesture land on the data under the cursor.
  return mapping_.to_device(x, y);
}

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 {
  // Made current first: the renderer's RAII handles issue `glDelete*` from their
  // destructors, and GTK clears the context in its own unrealize vfunc, which runs
  // after this signal's handlers -- so this is the last moment at which those
  // deletes can name a live context.
  gtk_gl_area_make_current(GTK_GL_AREA(widget_));
  // Released unconditionally. The area's error says nothing about whether the
  // renderer holds names to delete: it is set when context *creation* failed, and
  // that is the path `realize()` returns from before it builds a renderer, so a
  // renderer present here has GL objects behind it. Leaving it for the wrapper's
  // destructor -- which runs at widget finalisation, after GTK has freed the
  // context -- would issue those deletes with nothing current, which is exactly
  // what the old error check was trying to avoid. `make_current()` and `reset()`
  // are no-ops on a widget that never got a context and on an empty renderer.
  renderer_.reset();
  press_open_ = false;
  drag_.have = false;
  zoom_.end();
}

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);

  // The mapping's factor is the display's scale, so the renderer draws its logical
  // style metrics at the same ratio the viewport and every forwarded coordinate are
  // in: a 12 px label is the same physical size on a 1x and a 2x widget. It is the
  // `::resize` ratio, not `gtk_widget_get_scale_factor()` -- the latter is the
  // ceiling under fractional scaling and would over-scale the chrome by up to 60%.
  renderer_->style().ui_scale = mapping_.factor;
  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 {
  auto* area = static_cast<PlotArea*>(self);
  // GTK's two sizes, and the one place the mapping is derived from them. `width`
  // and `height` are the device pixels of the viewport this signal's default
  // handler is about to set -- `glViewport(0, 0, width, height)` in GTK's own
  // `gtk_gl_area_resize()` -- and the widget's width and height, which GTK has
  // already updated when it emits this, are the logical pixels that same rectangle
  // is reported in. Their ratio is the factor for the viewport *and* for every
  // forwarded coordinate; refreshing here means a gesture arriving between this
  // resize and the next frame maps against the new geometry rather than the old.
  area->mapping_ = DeviceMapping::from_sizes(
      Vec2{static_cast<f32>(gtk_widget_get_width(area->widget_)),
           static_cast<f32>(gtk_widget_get_height(area->widget_))},
      Vec2{static_cast<f32>(width), static_cast<f32>(height)});
  area->refresh_viewport();
}

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

void PlotArea::pointer_press(input::Button button, u32 mods, double x, double y,
                             u32 time_ms) 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;
  e.time_s = pointer_time_s(time_ms);  // the double-click detector differences these
  controller_.on_pointer(e);
  gtk_gl_area_queue_render(GTK_GL_AREA(widget_));
}

void PlotArea::pointer_release(Vec2 position, u32 time_ms) 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;
  e.time_s = pointer_time_s(time_ms);
  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,
                      gtk_event_controller_get_current_event_time(GTK_EVENT_CONTROLLER(gesture)));
}

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),
                        gtk_event_controller_get_current_event_time(GTK_EVENT_CONTROLLER(gesture)));
}

void PlotArea::on_click_stopped(GtkGestureClick* gesture, void* self) noexcept {
  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_.have) {
    area->pointer_release(
        area->pointer_,
        gtk_event_controller_get_current_event_time(GTK_EVENT_CONTROLLER(gesture)));
  }
}

void PlotArea::on_drag_begin(GtkGestureDrag* gesture, double start_x, double start_y,
                             void* self) noexcept {
  auto* area = static_cast<PlotArea*>(self);
  area->drag_.begin(static_cast<f32>(start_x), static_cast<f32>(start_y));
  area->pointer_ = area->drag_.position(area->mapping_);
  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,
                        gtk_event_controller_get_current_event_time(GTK_EVENT_CONTROLLER(gesture)));
  }
}

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;
  // The anchor stays logical (see `DragAnchor`): the gesture reports the offset
  // from the press point in logical pixels, and this is a logical difference
  // scaled by the factor in force *now*, so a factor change between two samples
  // cannot turn into a phantom movement of its own.
  const DragAnchor::Sample sample =
      area->drag_.update(offset_x, offset_y, area->mapping_);
  area->pointer_ = sample.position;
  area->have_pointer_ = true;

  input::PointerEvent e;
  e.x = sample.position.x;
  e.y = sample.position.y;
  e.dx = sample.delta.x;
  e.dy = sample.delta.y;
  e.button = area->press_button_;
  e.mods = area->press_mods_;
  e.down = true;
  e.time_s = area->pointer_time_s(
      gtk_event_controller_get_current_event_time(GTK_EVENT_CONTROLLER(gesture)));
  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 {
  auto* area = static_cast<PlotArea*>(self);
  // 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 offset `drag-end` itself reports -- which may be past the last update's --
  // and not the last update's position.
  const Vec2 position = area->drag_.end(offset_x, offset_y, area->mapping_);
  area->pointer_ = position;
  area->pointer_release(
      position, gtk_event_controller_get_current_event_time(GTK_EVENT_CONTROLLER(gesture)));
}

// --- the pinch ------------------------------------------------------------

void PlotArea::on_zoom_begin(GtkGestureZoom* gesture, GdkEventSequence*, void* self) noexcept {
  auto* area = static_cast<PlotArea*>(self);
  // The focal point is the centre of the bounding box around the pointers the
  // gesture holds, in logical widget pixels -- the same space every other
  // gesture coordinate here arrives in, and what `to_device()` maps.
  double cx = 0.0;
  double cy = 0.0;
  gtk_gesture_get_bounding_box_center(GTK_GESTURE(gesture), &cx, &cy);
  area->zoom_.begin(static_cast<f32>(gtk_gesture_zoom_get_scale_delta(gesture)),
                    Vec2{static_cast<f32>(cx), static_cast<f32>(cy)});
}

void PlotArea::on_zoom_scale_changed(GtkGestureZoom* gesture, double scale,
                                     void* self) noexcept {
  auto* area = static_cast<PlotArea*>(self);
  double cx = 0.0;
  double cy = 0.0;
  gtk_gesture_get_bounding_box_center(GTK_GESTURE(gesture), &cx, &cy);
  // The `scale` argument and `gtk_gesture_zoom_get_scale_delta()` are the same
  // cumulative number -- the zoom since `::begin` -- so the folded event is this
  // *frame's* zoom factor about the fingers' centre, plus the centre's movement,
  // which is the two-finger pan. An update without a begin folds to a no-op (see
  // `ZoomAnchor::update`), so a gesture whose start the host never saw zooms
  // nothing rather than zooming from a made-up anchor.
  const input::GestureEvent g = area->zoom_.update(
      static_cast<f32>(scale), Vec2{static_cast<f32>(cx), static_cast<f32>(cy)}, area->mapping_);
  area->controller_.on_gesture(g);
  gtk_gl_area_queue_render(GTK_GL_AREA(area->widget_));
}

void PlotArea::on_zoom_end(GtkGestureZoom*, GdkEventSequence*, void* self) noexcept {
  // No trailing event: the last `scale-changed` carried the gesture's final zoom,
  // and `end` only closes the folding so a straggler update zooms nothing.
  static_cast<PlotArea*>(self)->zoom_.end();
}

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;
  }

  Vec2 at{};
  if (!current_event_position(GTK_EVENT_CONTROLLER(scroll), area->mapping_, &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;
  e.time_s = PlotArea::pointer_time_s(
      gtk_event_controller_get_current_event_time(GTK_EVENT_CONTROLLER(motion)));
  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 whole translation lives in gtk_input.hpp (see `key_from`): the function
  // keys -- arrows, Escape -- are matched on the *keyval*, which is layout- and
  // level-independent for them, and the printable ones on the Unicode they decode
  // to, which is what makes `+` and shift+`=` (usually the same physical key) both
  // zoom in, and the keypad's plus and minus come along for free. `0` is "no plot
  // key": GTK and the window keep it.
  const int key = key_from(static_cast<u32>(keyval), static_cast<u32>(gdk_keyval_to_unicode(keyval)));
  if (key == 0) return FALSE;

  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