fix(plot,render,glfw): a hue-neutral crosshair, an em-dash placeholder, one layout per figure event, and a usable readout switch

adapters/glfw/glfw_host.cpp

@@ -212,15 +212,16 @@ bool Host::run(const RunConfig& cfg, const FrameFn& frame, Status* st) {
       // controller_at() or injected by a scripted run -- is hit-tested against the
       // geometry the last frame drew, and the two readers of a row's geometry cannot
       // disagree about it.
+      const std::vector<Rect>& rows = figure_->layout().rows;  // the one layout of this frame
       for (u32 i = 0; i < row_controllers_.size(); ++i) {
-        row_controllers_[i]->set_viewport(figure_->layout().rows[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();
+      sync_figure_crosshair(rows);
     } else {
       controller_.set_viewport(viewport);  // also Plot::set_viewport, then layout()
     }
@@ -430,20 +431,23 @@ 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 u32 row = row_at(x, y);
-  if (row >= row_controllers_.size()) return controller_;
+  // The figure's one layout for this call: both readers below take the row rects it produced, so an
+  // event lays the figure out once and not once per reader -- this is the hottest path a multi-row
+  // figure has, and a layout now rebuilds every row's ticks, grid and crosshair geometry.
+  const std::vector<Rect>& rows = figure_->layout().rows;
+  if (rows.empty() || rows.size() != row_controllers_.size()) return controller_;
+  (void)x;  // the rows tile the figure vertically, so y alone picks one -- see row_in()
+  const u32 row = row_in(rows, 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(figure_->layout().rows[row]);
+  controller.set_viewport(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;
+u32 Host::row_in(const std::vector<Rect>& rows, f32 y) noexcept {
+  if (rows.empty()) 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
@@ -462,13 +466,12 @@ u32 Host::row_at(f32 x, f32 y) const noexcept {
       if (distance == 0.0f) break;  // inside this row: nothing can be nearer
     }
   }
-  (void)x;  // the rows tile the figure vertically, so y alone picks one
   return row;
 }
 
-void Host::sync_figure_crosshair() noexcept {
-  const u32 rows = figure_ != nullptr ? figure_->row_count() : 0u;
-  if (rows == 0u || row_controllers_.empty()) return;
+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
@@ -477,18 +480,18 @@ void Host::sync_figure_crosshair() noexcept {
   // 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;
+  u32 hovered = row_count;
   if (have_pointer_) {
-    hovered = row_at(last_pointer_x_, last_pointer_y_);
+    hovered = row_in(rows, last_pointer_y_);
   } else {
-    for (u32 i = 0; i < rows; ++i) {
+    for (u32 i = 0; i < row_count; ++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;
+    if (hovered >= row_count) return;
   }
 
   // Applied to the whole figure: one x, every row. A row whose controller has *no* crosshair (the