properly update label, don't say 'active' if chroma=0

When color-mapping chroma is 0, the hue slider has no effect, so say
"active" only when either L != 50 or chroma != 0.

Update the section label inside the value-changed callback for
the items in the collapsible section so that we can immediately
see the update instead of having to wait until an image is
opened in darkroom view.

src/gui/gtk.c

@@ -4168,8 +4168,7 @@ void dt_gui_collapsible_section_set_label(dt_gui_collapsible_section_t *cs,
   if (!cs || !cs->label || !label)
     return;
   gtk_label_set_text(GTK_LABEL(cs->label), label);
-//  dt_control_queue_redraw_widget(cs->label);
-  dt_control_queue_redraw();
+  dt_control_queue_redraw_widget(cs->label);
 }
 
 gboolean dt_gui_long_click(const int second,

src/iop/channelmixerrgb.c

@@ -600,7 +600,6 @@ static gboolean _area_mapping_active(const dt_iop_channelmixer_rgb_gui_data_t *g
 {
   return g && g->spot_mode && dt_bauhaus_combobox_get(g->spot_mode) != DT_SPOT_MODE_MEASURE &&
     ((g->lightness_spot && dt_bauhaus_slider_get_val(g->lightness_spot) != 50.0f) ||
-     (g->hue_spot && dt_bauhaus_slider_get_val(g->hue_spot) != 0.0f) ||
      (g->chroma_spot && dt_bauhaus_slider_get_val(g->chroma_spot) != 0.0f));
 }
 
@@ -4013,6 +4012,9 @@ static void _spot_settings_changed_callback(GtkWidget *slider,
   dt_conf_set_float("darkroom/modules/channelmixerrgb/hue", Lch_target[2] * 360.f);
   dt_conf_set_bool("darkroom/modules/channelmixerrgb/use_mixing", use_mixing);
 
+  // update the "active" flag in the GUI
+  dt_gui_collapsible_section_set_label(&g->csspot, _area_mapping_section_text(g));
+
   ++darktable.gui->reset;
   _paint_hue(self);
   --darktable.gui->reset;