fix(plot,glfw): layer the crosshair under the handles, sync it across a figure, and harden the lookup

adapters/glfw/glfw_host.cpp

@@ -215,6 +215,12 @@ bool Host::run(const RunConfig& cfg, const FrameFn& frame, Status* st) {
       for (u32 i = 0; i < row_controllers_.size(); ++i) {
         row_controllers_[i]->set_viewport(figure_->layout().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();
     } else {
       controller_.set_viewport(viewport);  // also Plot::set_viewport, then layout()
     }
@@ -406,6 +412,10 @@ 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.
+  spread_active_ = false;
+  spread_x_ = 0.0;
   if (figure_ == nullptr) return;
   const u32 rows = figure_->row_count();
   row_controllers_.reserve(rows);
@@ -420,9 +430,20 @@ input::Controller* Host::row_controller(u32 row) noexcept {
 
 input::Controller& Host::controller_at(f32 x, f32 y) noexcept {
   if (figure_ == nullptr || row_controllers_.empty()) return controller_;
-  const FigureLayout& layout = figure_->layout();
-  const std::vector<Rect>& rows = layout.rows;
-  if (rows.empty() || rows.size() != row_controllers_.size()) return controller_;
+  const u32 row = row_at(x, y);
+  if (row >= row_controllers_.size()) return controller_;
+  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(figure_->layout().rows[row]);
+  return controller;
+}
+
+u32 Host::row_at(f32 x, f32 y) const noexcept {
+  if (figure_ == nullptr || row_controllers_.empty()) return 0u;
+  const std::vector<Rect>& rows = figure_->layout().rows;
+  if (rows.empty() || rows.size() != row_controllers_.size()) return 0u;
   // The row the pixel is in, or -- when it is in none of them, which is the row gap a
   // host has asked for with `Figure::set_row_gap()` or the figure's own margins -- the
   // row whose rect is nearest. Selection by distance rather than by "the first that
@@ -442,12 +463,46 @@ input::Controller& Host::controller_at(f32 x, f32 y) noexcept {
     }
   }
   (void)x;  // the rows tile the figure vertically, so y alone picks one
-  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;
+  return row;
+}
+
+void Host::sync_figure_crosshair() noexcept {
+  const u32 rows = figure_ != nullptr ? figure_->row_count() : 0u;
+  if (rows == 0u || row_controllers_.empty()) return;
+
+  // 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 = rows;
+  if (have_pointer_) {
+    hovered = row_at(last_pointer_x_, last_pointer_y_);
+  } else {
+    for (u32 i = 0; i < rows; ++i) {
+      const Crosshair& candidate = figure_->plot(i).crosshair();
+      if (candidate.active != spread_active_ || (candidate.active && candidate.x != spread_x_)) {
+        hovered = i;
+        break;
+      }
+    }
+    if (hovered >= rows) return;
+  }
+
+  // Applied to the whole figure: one x, every row. 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) {
+    figure_->set_crosshair(row.crosshair().x, true);
+    spread_x_ = row.crosshair().x;
+    spread_active_ = true;
+  } else {
+    figure_->clear_crosshair();
+    spread_active_ = false;
+  }
 }
 
 }  // namespace plot::glfw