UI: Support mouse over highlighting for superimposed icons

E.g. the 'x' icons or eyedropper icons in text buttons. They didn't use
to have any mouse over feedback, now we dim the icon until hovered.
This kind of feedback helps users see that the icons are interactive,
and if they are within their interaction hotspot.

source/blender/editors/interface/interface.c

@@ -767,6 +767,38 @@ uiBut *ui_but_find_new(uiBlock *block_new, const uiBut *but_old)
   return but_new;
 }
 
+static bool ui_but_extra_icons_equals_old(const uiButExtraOpIcon *new_extra_icon,
+                                          const uiButExtraOpIcon *old_extra_icon)
+{
+  return (new_extra_icon->optype_params->optype == old_extra_icon->optype_params->optype) &&
+         (new_extra_icon->icon == old_extra_icon->icon);
+}
+
+static uiButExtraOpIcon *ui_but_extra_icon_find_old(const uiButExtraOpIcon *new_extra_icon,
+                                                    const uiBut *old_but)
+{
+  LISTBASE_FOREACH (uiButExtraOpIcon *, op_icon, &old_but->extra_op_icons) {
+    if (ui_but_extra_icons_equals_old(new_extra_icon, op_icon)) {
+      return op_icon;
+    }
+  }
+  return NULL;
+}
+
+static void ui_but_extra_icons_update_from_old_but(const uiBut *new_but, const uiBut *old_but)
+{
+  /* Specifically for keeping some state info for the active button. */
+  BLI_assert(old_but->active);
+
+  LISTBASE_FOREACH (uiButExtraOpIcon *, new_extra_icon, &new_but->extra_op_icons) {
+    uiButExtraOpIcon *old_extra_icon = ui_but_extra_icon_find_old(new_extra_icon, old_but);
+    /* Keep the highlighting state, and let handling update it later. */
+    if (old_extra_icon) {
+      new_extra_icon->highlighted = old_extra_icon->highlighted;
+    }
+  }
+}
+
 /**
  * \return true when \a but_p is set (only done for active buttons).
  */
@@ -854,6 +886,7 @@ static bool ui_but_update_from_old_block(const bContext *C,
     oldbut->flag = (oldbut->flag & ~flag_copy) | (but->flag & flag_copy);
     oldbut->drawflag = (oldbut->drawflag & ~drawflag_copy) | (but->drawflag & drawflag_copy);
 
+    ui_but_extra_icons_update_from_old_but(but, oldbut);
     SWAP(ListBase, but->extra_op_icons, oldbut->extra_op_icons);
 
     if (oldbut->type == UI_BTYPE_SEARCH_MENU) {
@@ -1605,6 +1638,7 @@ static PointerRNA *ui_but_extra_operator_icon_add_ptr(uiBut *but,
   WM_operator_properties_create_ptr(extra_op_icon->optype_params->opptr,
                                     extra_op_icon->optype_params->optype);
   extra_op_icon->optype_params->opcontext = opcontext;
+  extra_op_icon->highlighted = false;
 
   BLI_addtail(&but->extra_op_icons, extra_op_icon);
 
@@ -1810,6 +1844,11 @@ void UI_block_end_ex(const bContext *C, uiBlock *block, const int xy[2], int r_x
 
   BLI_assert(block->active);
 
+  /* Extend button data. This needs to be done before the block updating. */
+  LISTBASE_FOREACH (uiBut *, but, &block->buttons) {
+    ui_but_predefined_extra_operator_icons_add(but);
+  }
+
   UI_block_update_from_old(C, block);
 
   /* inherit flags from 'old' buttons that was drawn here previous, based
@@ -1841,7 +1880,6 @@ void UI_block_end_ex(const bContext *C, uiBlock *block, const int xy[2], int r_x
     if (UI_but_is_decorator(but)) {
       ui_but_anim_decorate_update_from_flag((uiButDecorator *)but);
     }
-    ui_but_predefined_extra_operator_icons_add(but);
 
 #ifndef NDEBUG
     ui_but_validate(but);

source/blender/editors/interface/interface_handlers.c

@@ -4194,6 +4194,31 @@ static bool ui_do_but_extra_operator_icon(bContext *C,
   return false;
 }
 
+static void ui_do_but_extra_operator_icons_mousemove(uiBut *but,
+                                                     uiHandleButtonData *data,
+                                                     const wmEvent *event)
+{
+  uiButExtraOpIcon *old_highlighted = NULL;
+
+  /* Unset highlighting of all first. */
+  LISTBASE_FOREACH (uiButExtraOpIcon *, op_icon, &but->extra_op_icons) {
+    if (op_icon->highlighted) {
+      old_highlighted = op_icon;
+    }
+    op_icon->highlighted = false;
+  }
+
+  uiButExtraOpIcon *hovered = ui_but_extra_operator_icon_mouse_over_get(but, data, event);
+
+  if (hovered) {
+    hovered->highlighted = true;
+  }
+
+  if (old_highlighted != hovered) {
+    ED_region_tag_redraw_no_rebuild(data->region);
+  }
+}
+
 #ifdef USE_DRAG_TOGGLE
 /* Shared by any button that supports drag-toggle. */
 static bool ui_do_but_ANY_drag_toggle(
@@ -8709,6 +8734,9 @@ static int ui_handle_button_event(bContext *C, const wmEvent *event, uiBut *but)
           button_tooltip_timer_reset(C, but);
         }
 
+        /* Update extra icons states. */
+        ui_do_but_extra_operator_icons_mousemove(but, data, event);
+
         break;
       }
       case TIMER: {

source/blender/editors/interface/interface_intern.h

@@ -375,6 +375,8 @@ typedef struct uiButExtraOpIcon {
 
   BIFIconID icon;
   struct wmOperatorCallParams *optype_params;
+
+  bool highlighted;
 } uiButExtraOpIcon;
 
 typedef struct ColorPicker {

source/blender/editors/interface/interface_widgets.c

@@ -2248,10 +2248,15 @@ static void widget_draw_extra_icons(const uiWidgetColors *wcol,
   /* inverse order, from right to left. */
   LISTBASE_FOREACH_BACKWARD (uiButExtraOpIcon *, op_icon, &but->extra_op_icons) {
     rcti temp = *rect;
+    float alpha_this = alpha;
 
     temp.xmin = temp.xmax - (BLI_rcti_size_y(rect) * 1.08f);
 
-    widget_draw_icon(but, op_icon->icon, alpha, &temp, wcol->text);
+    if (!op_icon->highlighted) {
+      alpha_this *= 0.75f;
+    }
+
+    widget_draw_icon(but, op_icon->icon, alpha_this, &temp, wcol->text);
 
     rect->xmax -= ICON_SIZE_FROM_BUTRECT(rect);
   }