test(plot): pin the crosshair's lookup, readout, placement, sync and clip

src/render/renderer.cpp

@@ -487,6 +487,12 @@ void Renderer::draw_plot(Plot& plot, const PlotLayout& layout) {
   draw_axis_lines(plot);
   set_clip(layout.axes);
   draw_traces(plot, layout);
+  // The crosshair's lines and markers over the data, but *under* the handles a pointer can
+  // grab: a hovered or grabbed cursor marker or region bound is feedback for a gesture in
+  // progress, and the crosshair -- which is drawn wherever the pointer is, including on top of
+  // the handle being grabbed -- must not hide it. Its readout panel, which is a backplate for
+  // text rather than a stroke, is drawn last of all (see below).
+  draw_crosshair_lines(plot, layout);
   // Regions over the traces and under the cursors: an area of interest the data is
   // still legible through, with the markers that bound it on top of it.
   draw_regions(plot, layout);
@@ -494,10 +500,10 @@ void Renderer::draw_plot(Plot& plot, const PlotLayout& layout) {
   draw_measures(plot, layout);
   draw_annotations(plot);
   draw_legend(plot, layout);
-  // The crosshair last of the interior passes: it is what the pointer is on, so its panel is
-  // drawn over the legend and every other overlay rather than under them, and its text joins
-  // the frame's single flush with everything else.
-  draw_crosshair(plot, layout);
+  // The crosshair's readout last of the interior passes: it is what the pointer is on, so its
+  // panel is drawn over the legend and every other overlay rather than under them, and its text
+  // joins the frame's single flush with everything else.
+  draw_crosshair_readout(plot, layout);
   clear_clip();
   draw_tick_labels(plot, layout);
   draw_titles(plot, layout);
@@ -1174,7 +1180,7 @@ void Renderer::draw_legend(const Plot& plot, const PlotLayout& layout) {
   }
 }
 
-void Renderer::draw_crosshair(const Plot& plot, const PlotLayout& layout) {
+void Renderer::draw_crosshair_lines(const Plot& plot, const PlotLayout& layout) {
   const CrosshairLayout& crosshair = layout.crosshair;
   if (!crosshair.active) return;
 
@@ -1213,6 +1219,11 @@ void Renderer::draw_crosshair(const Plot& plot, const PlotLayout& layout) {
     lines_.add_segment({row.marker.x, row.marker.y - arm}, {row.marker.x, row.marker.y + arm});
     flush_lines(pixel_mapping(), color, style_.device(1.0f));
   }
+}
+
+void Renderer::draw_crosshair_readout(const Plot& plot, const PlotLayout& layout) {
+  const CrosshairLayout& crosshair = layout.crosshair;
+  if (!crosshair.active) return;
 
   // The readout: the same backplate a legend has -- the frame's background at a partial
   // alpha, so the data stays legible through it -- with its border in the crosshair's own