plotter/adapters/glfw/glfw_host.cpp

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

#include <glfw/glfw_host.hpp>

// Before GLFW's header, so it does not include <GL/gl.h>: that header's macros
// (GL_COLOR_BUFFER_BIT and friends) would rewrite `plot::gl`'s enumerators of the
// same name. The adapter needs GLFW for the window, the context and the events
// only -- every GL entry point comes from plot::gl::load().
#define GLFW_INCLUDE_NONE
#include <GLFW/glfw3.h>

#include <plot/gl/loader.hpp>

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <limits>

namespace plot::glfw {
namespace {

/// GLFW_MOD_* and `plot::input::Mod` are the same bits, by design: the
/// controller documents `Mod` as the host-translatable mask. A key event's mask
/// therefore passes through, and only a pointer or wheel event, whose callbacks
/// carry no mask, has to ask the window for the current key state.
static_assert(static_cast<u32>(GLFW_MOD_SHIFT) == static_cast<u32>(input::Mod::Shift));
static_assert(static_cast<u32>(GLFW_MOD_CONTROL) == static_cast<u32>(input::Mod::Ctrl));
static_assert(static_cast<u32>(GLFW_MOD_ALT) == static_cast<u32>(input::Mod::Alt));
static_assert(static_cast<u32>(GLFW_MOD_SUPER) == static_cast<u32>(input::Mod::Super));

u32 mods_now(GLFWwindow* w) noexcept {
  u32 m = 0;
  if (glfwGetKey(w, GLFW_KEY_LEFT_SHIFT) == GLFW_PRESS ||
      glfwGetKey(w, GLFW_KEY_RIGHT_SHIFT) == GLFW_PRESS) {
    m |= GLFW_MOD_SHIFT;
  }
  if (glfwGetKey(w, GLFW_KEY_LEFT_CONTROL) == GLFW_PRESS ||
      glfwGetKey(w, GLFW_KEY_RIGHT_CONTROL) == GLFW_PRESS) {
    m |= GLFW_MOD_CONTROL;
  }
  if (glfwGetKey(w, GLFW_KEY_LEFT_ALT) == GLFW_PRESS ||
      glfwGetKey(w, GLFW_KEY_RIGHT_ALT) == GLFW_PRESS) {
    m |= GLFW_MOD_ALT;
  }
  if (glfwGetKey(w, GLFW_KEY_LEFT_SUPER) == GLFW_PRESS ||
      glfwGetKey(w, GLFW_KEY_RIGHT_SUPER) == GLFW_PRESS) {
    m |= GLFW_MOD_SUPER;
  }
  return m;
}

bool any_button_down(GLFWwindow* w) noexcept {
  return glfwGetMouseButton(w, GLFW_MOUSE_BUTTON_LEFT) == GLFW_PRESS ||
         glfwGetMouseButton(w, GLFW_MOUSE_BUTTON_RIGHT) == GLFW_PRESS ||
         glfwGetMouseButton(w, GLFW_MOUSE_BUTTON_MIDDLE) == GLFW_PRESS;
}

/// `glfwGetProcAddress` returns GLFW's `void (*)(void)`, `plot::gl::ProcLoader`
/// returns `void*`: a function pointer respelled as an object pointer. POSIX
/// guarantees the two have the same representation, and the bit copy states that
/// where a `reinterpret_cast` would be merely conditionally supported.
void* resolve_gl(const char* name) noexcept {
  GLFWglproc p = glfwGetProcAddress(name);
  void* out = nullptr;
  static_assert(sizeof(out) == sizeof(p), "function and object pointers must match");
  std::memcpy(&out, &p, sizeof(out));
  return out;
}

bool fail(Status* st, ErrorCode code, const char* message) noexcept {
  if (st != nullptr) {
    st->code = code;
    st->message = message;
  }
  return false;
}

/// `PLOT_DEMO_EXIT_AFTER_MS` as seconds: 0.0 when unset, unparsable or not
/// positive, which is "run until the window is closed".
f64 exit_after_seconds() noexcept {
  const char* v = std::getenv("PLOT_DEMO_EXIT_AFTER_MS");
  if (v == nullptr || v[0] == '\0') return 0.0;
  char* end = nullptr;
  const f64 ms = std::strtod(v, &end);
  if (end == v || !(ms > 0.0)) return 0.0;
  return ms * 1.0e-3;
}

}  // namespace

Host::Host() noexcept : controller_(plot_) {}

Host::~Host() { close(); }

void Host::close() noexcept {
  // The renderer's RAII handles call glDelete* from their destructors, so the
  // context has to be current -- and alive -- when they run.
  if (renderer_) {
    if (window_ != nullptr) glfwMakeContextCurrent(window_);
    renderer_.reset();
  }
  if (window_ != nullptr) {
    glfwDestroyWindow(window_);
    window_ = nullptr;
  }
  glfwMakeContextCurrent(nullptr);
  glfwTerminate();
  // The figure and its row controllers are deliberately *not* dropped here: they are the
  // caller's state, created by set_figure(), and a second run() reuses them (the run loop
  // hands each row its rect every frame). Dropping them would silently stop routing
  // gestures to the rows on the second run.
  have_pointer_ = false;
  // The cached rows described a figure's geometry; the next run() re-reads them from
  // scratch, and until then `controller_at()` answers the single-plot controller
  // rather than rects from a window that no longer exists.
  row_rects_.clear();
  // A closed host reports no size: a second run() must not see the previous
  // window's framebuffer size before its first refresh_sizes().
  fb_width_ = 0;
  fb_height_ = 0;
  fb_scale_x_ = 1.0f;
  fb_scale_y_ = 1.0f;
}

bool Host::run(const RunConfig& cfg, const FrameFn& frame, Status* st) {
  if (st != nullptr) *st = Status{};
  close();  // a second run() starts from a clean window

  if (glfwInit() != GLFW_TRUE) {
    return fail(st, ErrorCode::Unsupported, "glfwInit() failed");
  }

  glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
  glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
  glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
  // Required on macOS 10.7+, ignored elsewhere.
  glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GLFW_TRUE);
  // A run with a deadline is the automated path, so it stays off-screen: a
  // bounded CI run must never flash a window on a real display.
  const f64 exit_after_s = exit_after_seconds();
  if (exit_after_s > 0.0) glfwWindowHint(GLFW_VISIBLE, GLFW_FALSE);

  GLFWwindow* w = glfwCreateWindow(cfg.width > 0 ? cfg.width : 1,
                                   cfg.height > 0 ? cfg.height : 1,
                                   cfg.title != nullptr ? cfg.title : "plot", nullptr, nullptr);
  if (w == nullptr) {
    const char* desc = nullptr;
    (void)glfwGetError(&desc);
    std::fprintf(stderr, "plot::glfw: glfwCreateWindow() failed: %s\n",
                 desc != nullptr ? desc : "no error description");
    close();
    return fail(st, ErrorCode::Unsupported,
                "glfwCreateWindow() failed: no OpenGL 3.3 core context (see stderr)");
  }

  window_ = w;
  glfwSetWindowUserPointer(w, this);
  glfwMakeContextCurrent(w);
  glfwSwapInterval(cfg.vsync ? 1 : 0);
  glfwSetCursorPosCallback(w, &Host::cursor_pos_cb);
  glfwSetMouseButtonCallback(w, &Host::mouse_button_cb);
  glfwSetScrollCallback(w, &Host::scroll_cb);
  glfwSetKeyCallback(w, &Host::key_cb);
  glfwSetFramebufferSizeCallback(w, &Host::framebuffer_size_cb);
  refresh_sizes();

  if (!plot::gl::load(&resolve_gl)) {
    close();
    return fail(st, ErrorCode::Unsupported,
                "plot::gl::load() failed: the context is missing a required OpenGL 3.3 entry point");
  }

  renderer_.emplace();
  if (!renderer_->init(st)) {
    close();
    if (st != nullptr && st->code == ErrorCode::Ok) {
      *st = Status{ErrorCode::GLFailure, "Renderer::init() failed"};
    }
    return false;
  }

  const char* const screenshot = std::getenv("PLOT_DEMO_SCREENSHOT");
  const f64 start = glfwGetTime();
  f64 previous = start;

  while (glfwWindowShouldClose(window_) == GLFW_FALSE) {
    glfwPollEvents();
    refresh_sizes();

    if (fb_width_ == 0 || fb_height_ == 0) {
      // Minimised, or a size the compositor reports as zero: nothing to draw
      // into, and nothing to hand `begin_frame` -- a stale larger size would be
      // a lie about the framebuffer. Idle on events rather than spin, and still
      // honour the exit deadline here: a bounded run that never gets a drawable
      // must end, not idle past the deadline it was given. There is no frame to
      // dump in this state, so the screenshot is skipped.
      if (exit_after_s > 0.0 && glfwGetTime() - start >= exit_after_s) {
        std::fprintf(stderr,
                     "[glfw] exit deadline reached with no drawable (framebuffer 0x0);"
                     " nothing to dump\n");
        break;
      }
      glfwWaitEventsTimeout(0.05);
      continue;
    }

    // The plot's region is the whole window here, which is what the controller
    // and `begin_frame` must agree on: one rect, in framebuffer pixels, set
    // before the frame callback runs so it sees the layout it draws into.
    const Rect viewport{0.0f, 0.0f, static_cast<f32>(fb_width_), static_cast<f32>(fb_height_)};
    if (figure_ != nullptr) {
      // Figure mode: the rows tile the whole framebuffer, and it is `Figure::layout()`
      // that hands each row its rect (and so its own `Plot` its viewport). The single
      // controller is untouched -- each row has its own, see controller_at().
      figure_->set_viewport(viewport);
      // The frame's one layout of the figure, into the cache every event handler
      // between now and the next frame reads (R7): the rows below, and the routing of
      // whatever the next `glfwPollEvents()` batch delivers, all take *these* rects
      // rather than each paying for another layout.
      refresh_row_rects();
      const std::vector<Rect>& rows = row_rects_;
      for (u32 i = 0; i < row_controllers_.size(); ++i) {
        if (i < rows.size()) row_controllers_[i]->set_viewport(rows[i]);
      }
      // One hover, every row (spec §15): the crosshair the row controller under the pointer
      // placed is applied to the whole figure here, after the pointers of this frame have been
      // handled and before anything lays the rows out -- so every row's readout lists its own
      // signals at the one x the pointer is over. In the run loop rather than in a callback, so a
      // scripted run that injects its events straight into a row controller is synced too.
      sync_figure_crosshair(rows);
      // Only the row the pointer's events go to ever republishes its own hot handles, so a row
      // the pointer left keeps the `HotHandles` it last published and goes on drawing a hovered
      // marker or region bound as if the pointer were still on it -- and its `hovered_region()`/
      // `grabbed_cursor()` keep answering for it. Clear every *other* row here, in the run loop,
      // where that row is known (the captured row while a drag owns the button, so an in-flight
      // gesture keeps its highlight).
      if (have_pointer_ && !rows.empty()) {
        const u32 active = pointer_row(rows);
        for (u32 i = 0; i < rows.size() && i < row_controllers_.size(); ++i) {
          if (i != active) figure_->plot(i).set_hot_handles(HotHandles{});
        }
      }
    } else {
      controller_.set_viewport(viewport);  // also Plot::set_viewport, then layout()
    }

    const f64 now = glfwGetTime();
    const f64 dt = now - previous;
    previous = now;
    // A figure's rows take the pointer, but the frame callback still has to be given a
    // `Plot&`: row 0 is the one it gets, so the callback argument and `figure()->plot(0)`
    // are the same plot.
    if (frame) frame(figure_ != nullptr ? figure_->plot(0u) : plot_, dt);

    const bool exiting = exit_after_s > 0.0 && (now - start) >= exit_after_s;

    renderer_->begin_frame(fb_width_, fb_height_, viewport);
    if (figure_ != nullptr) {
      renderer_->draw(*figure_);
    } else {
      renderer_->draw(plot_);
    }
    if (exiting && screenshot != nullptr && screenshot[0] != '\0') {
      Status shot;
      const bool dumped = renderer_->dump_frame_ppm(screenshot, &shot);
      std::printf("[glfw] screenshot %s: %s\n", screenshot,
                  dumped ? "written" : shot.message);
      std::fflush(stdout);
    }
    renderer_->end_frame();
    glfwSwapBuffers(window_);

    if (exiting) break;
  }
  return true;
}

void Host::set_title(const char* title) noexcept {
  if (window_ != nullptr) glfwSetWindowTitle(window_, title != nullptr ? title : "");
}

void Host::refresh_sizes() noexcept {
  if (window_ == nullptr) return;
  int fw = 0;
  int fh = 0;
  int ww = 0;
  int wh = 0;
  glfwGetFramebufferSize(window_, &fw, &fh);
  glfwGetWindowSize(window_, &ww, &wh);
  float sx = 1.0f;
  float sy = 1.0f;
  glfwGetWindowContentScale(window_, &sx, &sy);
  content_scale_x_ = sx;
  content_scale_y_ = sy;
  // Assigned unconditionally, zero included: an iconified window really does
  // report a 0x0 framebuffer (Windows, and several Linux window managers), and
  // the loop's no-drawable guard reads exactly this. Only ever writing a
  // positive size would leave the guard unreachable and keep handing
  // `begin_frame` a stale size for a framebuffer that is no longer that big.
  fb_width_ = fw > 0 ? static_cast<u32>(fw) : 0u;
  fb_height_ = fh > 0 ? static_cast<u32>(fh) : 0u;
  if (ww > 0 && fw > 0) fb_scale_x_ = static_cast<f32>(fw) / static_cast<f32>(ww);
  if (wh > 0 && fh > 0) fb_scale_y_ = static_cast<f32>(fh) / static_cast<f32>(wh);
  // The renderer draws its logical style metrics at this same ratio, so the chrome
  // is the same physical size at any scale: one value states the display, and a
  // 12 px label is 24 device px on the 2x panel both these sizes describe. Read here
  // rather than in the frame loop because this is the one place the two sizes are,
  // and re-read whenever the window moves to a display with another scale.
  if (renderer_) renderer_->style().ui_scale = fb_scale_x_;
}

// ------------------------------------------------------------------ events

Host* Host::from(GLFWwindow* w) noexcept {
  return w != nullptr ? static_cast<Host*>(glfwGetWindowUserPointer(w)) : nullptr;
}

void Host::cursor_pos_cb(GLFWwindow* w, double x, double y) {
  if (Host* h = from(w)) h->on_cursor_pos(x, y);
}

void Host::mouse_button_cb(GLFWwindow* w, int button, int action, int mods) {
  if (Host* h = from(w)) h->on_mouse_button(button, action, mods);
}

void Host::scroll_cb(GLFWwindow* w, double dx, double dy) {
  if (Host* h = from(w)) h->on_scroll(dx, dy);
}

void Host::key_cb(GLFWwindow* w, int key, int scancode, int action, int mods) {
  (void)scancode;
  if (Host* h = from(w)) h->on_key(key, action, mods);
}

void Host::framebuffer_size_cb(GLFWwindow* w, int width, int height) {
  (void)width;
  (void)height;
  if (Host* h = from(w)) h->refresh_sizes();
}

void Host::on_cursor_pos(f64 wx, f64 wy) noexcept {
  // Window coordinates -> framebuffer pixels: the same translation the viewport
  // and every other Rect here use, so a gesture lands where it was aimed.
  const f32 x = static_cast<f32>(wx) * fb_scale_x_;
  const f32 y = static_cast<f32>(wy) * fb_scale_y_;
  input::PointerEvent e;
  e.x = x;
  e.y = y;
  if (have_pointer_) {
    e.dx = x - last_pointer_x_;
    e.dy = y - last_pointer_y_;
  }
  last_pointer_x_ = x;
  last_pointer_y_ = y;
  have_pointer_ = true;
  e.mods = mods_now(window_);
  e.down = any_button_down(window_);
  e.time_s = glfwGetTime();  // GLFW's monotonic clock; only differences are read
  feed_pointer_event(e);
}

void Host::on_mouse_button(int button, int action, int mods) noexcept {
  input::Button b;
  switch (button) {
    case GLFW_MOUSE_BUTTON_LEFT: b = input::Button::Left; break;
    case GLFW_MOUSE_BUTTON_MIDDLE: b = input::Button::Middle; break;
    case GLFW_MOUSE_BUTTON_RIGHT: b = input::Button::Right; break;
    default: return;  // the controller has no gesture for the auxiliary buttons
  }
  double wx = 0.0;
  double wy = 0.0;
  glfwGetCursorPos(window_, &wx, &wy);
  input::PointerEvent e;
  e.x = static_cast<f32>(wx) * fb_scale_x_;
  e.y = static_cast<f32>(wy) * fb_scale_y_;
  // The press/release is also a pointer sample: the next move's delta starts
  // here, or a drag's first motion would include the jump to the click.
  last_pointer_x_ = e.x;
  last_pointer_y_ = e.y;
  have_pointer_ = true;
  e.button = b;
  e.mods = static_cast<u32>(mods);
  e.pressed = action == GLFW_PRESS;
  e.released = action == GLFW_RELEASE;
  e.down = any_button_down(window_);
  e.time_s = glfwGetTime();  // the double-click detector reads differences of these
  feed_pointer_event(e);
}

void Host::on_scroll(f64 dx, f64 dy) noexcept {
  double wx = 0.0;
  double wy = 0.0;
  glfwGetCursorPos(window_, &wx, &wy);
  input::WheelEvent e;
  e.x = static_cast<f32>(wx) * fb_scale_x_;
  e.y = static_cast<f32>(wy) * fb_scale_y_;
  e.dx = static_cast<f32>(dx);
  e.dy = static_cast<f32>(dy);  // GLFW's positive y (scroll up) is the zoom-in sign
  e.mods = mods_now(window_);
  controller_at(e.x, e.y).on_wheel(e);
}

void Host::on_key(int key, int action, int mods) noexcept {
  int mapped = 0;
  switch (key) {
    // The controller zooms on '+', which on most layouts is shift+'='; the '='
    // key is mapped to it as well, so both spellings zoom in.
    case GLFW_KEY_EQUAL:
    case GLFW_KEY_KP_ADD: mapped = input::kKeyZoomIn; break;
    case GLFW_KEY_MINUS:
    case GLFW_KEY_KP_SUBTRACT: mapped = input::kKeyZoomOut; break;
    case GLFW_KEY_F: mapped = input::kKeyFit; break;
    // Arrows pan (10% of the span per press, repeats included) and Escape cancels
    // the active drag: translated, exactly like the ASCII set above, and the
    // controller decides what each key does. Escape no longer closes the window --
    // the window manager's close button and Alt+F4 are the way to quit, and a key
    // the controller has a gesture for must reach it.
    case GLFW_KEY_ESCAPE: mapped = input::kKeyEscape; break;
    case GLFW_KEY_LEFT: mapped = input::kKeyLeft; break;
    case GLFW_KEY_RIGHT: mapped = input::kKeyRight; break;
    case GLFW_KEY_UP: mapped = input::kKeyUp; break;
    case GLFW_KEY_DOWN: mapped = input::kKeyDown; break;
    default: return;
  }
  input::KeyEvent e;
  e.key = mapped;
  e.mods = static_cast<u32>(mods);
  // GLFW reports auto-repeat as its own action; a held key keeps acting, exactly
  // as holding the key on a keyboard does.
  e.down = action != GLFW_RELEASE;
  // A key has no position of its own: it goes to the row the pointer was last over,
  // which is the row the user is looking at.
  input::Controller& controller =
      (figure_ == nullptr || last_row_ >= row_controllers_.size()) ? controller_
                                                                  : *row_controllers_[last_row_];
  controller.on_key(e);
}

void Host::set_figure(Figure* figure) noexcept {
  figure_ = figure;
  row_controllers_.clear();
  last_row_ = 0u;
  // A new figure holds no crosshair this host has spread: the comparison in
  // `sync_figure_crosshair()` starts from "nothing spread", which is the state its rows are in.
  // Nor does it hold a captured drag row: the controllers are rebuilt below, so the old
  // capture names a controller that no longer exists.
  spread_active_ = false;
  spread_x_ = 0.0;
  spread_y_ = 0.0;
  captured_ = false;
  captured_row_ = 0u;
  captured_button_ = input::Button::Left;
  if (figure_ == nullptr) {
    row_rects_.clear();  // no figure, no rows: the handlers fall back to `controller_`
    return;
  }
  // One controller per **plot** (a row of several cells has one for each), indexed by the
  // flattened plot index the figure uses.
  const u32 plots = figure_->plot_count();
  row_controllers_.reserve(plots);
  for (u32 i = 0; i < plots; ++i) {
    row_controllers_.push_back(std::make_unique<input::Controller>(figure_->plot(i)));
  }
  // The new figure's rects go into the cache now, not at the next frame: an event
  // arriving between set_figure() and the first frame -- a scripted run, the headless
  // tests -- must be routed against the figure it was just handed, and the next
  // `refresh_row_rects()` re-reads them once per frame as usual.
  refresh_row_rects();
}

void Host::refresh_row_rects() noexcept {
  // The one `Figure::layout()` per call: every event handler reads `row_rects_`
  // instead of paying this again, which is the whole point of the cache (R7). The rects
  // are the figure's **cells** (one per plot, flattened), which for a figure with one
  // cell per row are exactly its row rects.
  row_rects_ = figure_ != nullptr ? figure_->layout().cells : std::vector<Rect>{};
}

input::Controller* Host::row_controller(u32 row) noexcept {
  return row < row_controllers_.size() ? row_controllers_[row].get() : nullptr;
}

input::Controller& Host::controller_at(f32 x, f32 y) noexcept {
  if (figure_ == nullptr || row_controllers_.empty()) return controller_;
  // The cached rects `refresh_row_rects()` filled -- the same ones the last frame
  // drew -- and not a fresh `Figure::layout()`: an event pays no layout, which is
  // what made a burst of pointer moves on a multi-row figure re-lay every row out
  // per move (R7). Between a resize and the loop's next refresh the cache is one
  // frame stale, which is the documented worst case.
  const std::vector<Rect>& rows = row_rects_;
  if (rows.empty() || rows.size() != row_controllers_.size()) return controller_;
  const u32 row = row_in(rows, x, y);
  last_row_ = row;
  input::Controller& controller = *row_controllers_[row];
  // The row's own viewport, so the geometry this controller hit-tests is the geometry
  // the last frame drew: the same rect `Figure::layout()` gave the row's plot.
  controller.set_viewport(rows[row]);
  return controller;
}

void Host::feed_pointer_event(const input::PointerEvent& e) noexcept {
  if (figure_ == nullptr) {
    controller_.on_pointer(e);
    return;
  }
  pointer_target(e.x, e.y, e.button, e.pressed, e.released).on_pointer(e);
}

input::Controller& Host::pointer_target(f32 x, f32 y, input::Button button, bool pressed,
                                        bool released) noexcept {
  if (figure_ == nullptr || row_controllers_.empty()) return controller_;
  const std::vector<Rect>& rows = row_rects_;
  if (rows.empty() || rows.size() != row_controllers_.size()) return controller_;

  u32 row = 0u;
  if (captured_) {
    // A button-down gesture belongs to the row it began in for its whole life: a drag
    // that crosses into a neighbouring row must not hand its motion -- or its release --
    // to a controller that never saw the press, which would leave the owner's drag open
    // for ever (and let a later gesture resume it from the old anchor).
    row = captured_row_;
    // Only the owning button's release ends the capture (R5): a second button's
    // release -- pressed meanwhile as a chord -- must not strand the first button's
    // drag, and its press must not steal the capture to the row it landed in.
    if (released && button == captured_button_) captured_ = false;
  } else {
    row = row_in(rows, x, y);
    if (pressed) {
      captured_ = true;
      captured_row_ = row;
      captured_button_ = button;
    }
  }
  // The captured row is re-read against this frame's rects: the figure can be resized
  // mid-drag, and the guard keeps a stale index from reaching past the controller list.
  if (row >= row_controllers_.size()) row = 0u;
  last_row_ = row;
  input::Controller& controller = *row_controllers_[row];
  controller.set_viewport(rows[row]);
  return controller;
}

u32 Host::pointer_row(const std::vector<Rect>& rows) noexcept {
  if (captured_ && captured_row_ < rows.size()) return captured_row_;
  return row_in(rows, last_pointer_x_, last_pointer_y_);
}

u32 Host::row_in(const std::vector<Rect>& rows, f32 x, f32 y) noexcept {
  if (rows.empty()) return 0u;
  // The cell the pixel is in, or -- when it is in none of them, which is the row gap or
  // column gap a host has asked for with `Figure::set_row_gap()`/`set_col_gap()` or the
  // figure's own margins -- the cell whose rect is nearest (squared distance from the
  // pixel to the rect). Selection by distance rather than by "the first that matches" is
  // what makes a pixel in a gap belong to the cell it is *near*, and a gesture in a
  // visible gap must not be routed to plot 0 by default. For a figure with one cell per
  // row every cell spans the figure's width, so this reduces to the nearest row by y.
  u32 row = 0u;
  double best = std::numeric_limits<double>::infinity();
  for (u32 i = 0; i < rows.size(); ++i) {
    const Rect& r = rows[i];
    // In double: in f32 the dy^2 term can be swallowed by a large dx^2 (pointer far outside).
    const double dx = x < r.x ? double(r.x) - x : (x >= r.right() ? double(x) - r.right() : 0.0);
    const double dy = y < r.y ? double(r.y) - y : (y >= r.bottom() ? double(y) - r.bottom() : 0.0);
    const double distance = dx * dx + dy * dy;
    if (distance < best) {
      best = distance;
      row = i;
      if (distance == 0.0) break;  // inside this cell: nothing can be nearer
    }
  }
  return row;
}

void Host::sync_figure_crosshair(const std::vector<Rect>& rows) noexcept {
  if (rows.empty() || row_controllers_.empty()) return;
  const u32 row_count = static_cast<u32>(rows.size());

  // Which row's crosshair is the hover. With a pointer of the host's own it is the row that pointer
  // is over -- the same rule events are routed by. A scripted run injects its events straight into
  // a row controller and the host never sees a pointer, so there the hover is the row whose
  // crosshair is *not* the one this host spread last: the figure writes one value to every row, so
  // any row holding something else was changed from outside, and that is the one the run touched.
  // Nothing differing means every row still holds the last spread -- no new hover -- and the figure
  // keeps it.
  u32 hovered = row_count;
  if (have_pointer_) {
    // The row the pointer's events currently go to -- the captured row while a drag owns
    // the button, the hovered row otherwise -- so the hover follows the gesture it began.
    hovered = pointer_row(rows);
  } else {
    for (u32 i = 0; i < row_count; ++i) {
      const Crosshair& candidate = figure_->plot(i).crosshair();
      // A row differs when its active state, its x, or the y of *its own* horizontal line
      // differs from what was spread: a pointer that moves to another row at the same x is
      // a new hover and must move the one horizontal line, not leave the old row's behind.
      const bool y_differs = candidate.horizontal && candidate.y != spread_y_;
      if (candidate.active != spread_active_ ||
          (candidate.active && (candidate.x != spread_x_ || y_differs))) {
        hovered = i;
        break;
      }
    }
    if (hovered >= row_count) return;
  }

  // Applied to the whole figure: one x, every row; and the pointer row's **own** y, only to that
  // row -- so the row under the pointer gets the one horizontal line, and no row ever shows
  // another row's y (spec §17). The y is the row's own data coordinate, placed by its controller
  // from the pointer's pixel row, and it is never spread to a sibling. A row whose controller has
  // *no* crosshair (the pointer left its axes, which is the only thing that clears one) clears the
  // figure's, so the rows cannot be left showing a hover the pointer has abandoned.
  const Plot& row = figure_->plot(hovered);
  if (row.crosshair().active) {
    if (row.crosshair().horizontal) {
      // The row carries its own y (its controller placed both coordinates), so the figure's
      // four-argument overload gives the pointer row the one horizontal line.
      figure_->set_crosshair(row.crosshair().x, hovered, row.crosshair().y, true);
    } else {
      // The row's crosshair is vertical-only -- a caller placed it through the
      // single-coordinate `Figure::set_crosshair()`, which is documented vertical-only. The
      // four-argument overload would turn the horizontal line on at the stored (meaningless)
      // y == 0, clamped to the axes edge and made permanent. Promote only the vertical line.
      figure_->set_crosshair(row.crosshair().x, true);
    }
    spread_x_ = row.crosshair().x;
    spread_y_ = row.crosshair().y;
    spread_active_ = true;
  } else {
    figure_->clear_crosshair();
    spread_active_ = false;
  }
}

}  // namespace plot::glfw