libsysprof-gtk: add informative description of events under pointer

We are not using Tooltips for these because they 1) introduce additional
delay, 2) slow things down to a hault do to protocol overhead/compositor
work/etc.

There may be a desire to do this differently for a11y purposes at some
point, but I'd want to see how that use case plays out before trying to
shove text description updates every frame motion to a screen reader.

src/libsysprof-gtk/sysprof-time-span-layer.c

@@ -241,6 +241,66 @@ sysprof_time_span_layer_snapshot (GtkWidget   *widget,
     }
 }
 
+static gpointer
+sysprof_time_span_layer_lookup_item (SysprofChartLayer *layer,
+                                     double             x,
+                                     double             y)
+{
+  SysprofTimeSpanLayer *self = (SysprofTimeSpanLayer *)layer;
+  const float *x_values;
+  const float *x2_values;
+  GListModel *model;
+  guint n_x_values = 0;
+  guint n_x2_values = 0;
+  guint n_values;
+  int width;
+  int height;
+
+  g_assert (SYSPROF_IS_TIME_SPAN_LAYER (self));
+
+  width = gtk_widget_get_width (GTK_WIDGET (self));
+  height = gtk_widget_get_height (GTK_WIDGET (self));
+
+  if (width == 0 || height == 0 || self->series == NULL)
+    return NULL;
+
+  if (!(model = sysprof_series_get_model (SYSPROF_SERIES (self->series))))
+    return NULL;
+
+  if (!(x_values = sysprof_normalized_series_get_values (self->normal_x, &n_x_values)) ||
+      !(x2_values = sysprof_normalized_series_get_values (self->normal_x2, &n_x2_values)) ||
+      !(n_values = MIN (n_x_values, n_x2_values)))
+    return NULL;
+
+  /* First match our non-duration marks (diamonds) */
+  for (guint i = 0; i < n_values; i++)
+    {
+      int begin = x_values[i] * width - 3;
+      int end = x2_values[i] * width + 3;
+
+      if (x_values[i] != x2_values[i])
+        continue;
+
+      if (x >= begin && x < end)
+        return g_list_model_get_item (model, i);
+    }
+
+  /* Then match regular duration events */
+  for (guint i = 0; i < n_values; i++)
+    {
+      float begin = x_values[i] * width;
+      float end = x2_values[i] * width;
+
+      if (x_values[i] == x2_values[i])
+        continue;
+
+      if (x >= begin && x < end)
+        return g_list_model_get_item (model, i);
+    }
+
+  return NULL;
+}
+
 static void
 sysprof_time_span_layer_finalize (GObject *object)
 {
@@ -321,6 +381,7 @@ sysprof_time_span_layer_class_init (SysprofTimeSpanLayerClass *klass)
 {
   GObjectClass *object_class = G_OBJECT_CLASS (klass);
   GtkWidgetClass *widget_class = GTK_WIDGET_CLASS (klass);
+  SysprofChartLayerClass *chart_layer_class = SYSPROF_CHART_LAYER_CLASS (klass);
 
   object_class->finalize = sysprof_time_span_layer_finalize;
   object_class->get_property = sysprof_time_span_layer_get_property;
@@ -328,6 +389,8 @@ sysprof_time_span_layer_class_init (SysprofTimeSpanLayerClass *klass)
 
   widget_class->snapshot = sysprof_time_span_layer_snapshot;
 
+  chart_layer_class->lookup_item = sysprof_time_span_layer_lookup_item;
+
   properties[PROP_AXIS] =
     g_param_spec_object ("axis", NULL, NULL,
                          SYSPROF_TYPE_AXIS,