Fix #146993: Select tree view elements in popup menu

It seems `ui_handler_region_menu/ui_handle_menu_event` doesn't handle
any events other than press/double-click as they return `WM_UI_HANDLER_BREAK`.
`KM_CLICK` event is not possible due to above case. i.e. clicked element
can't be set active inside `ui_handle_view_item_event()`. (Guess `HANDLER_BREAK` also
prevents from looping over keymap to find relevant keyitem pair to invoke the operator?
Didn't check in details). To fix this, add a condition that checks whether button is in
popup menu. If yes, force active the pressed button and keep the popup open.

Pull Request: https://projects.blender.org/blender/blender/pulls/147047

source/blender/editors/interface/interface_handlers.cc

@@ -5150,6 +5150,12 @@ static int ui_do_but_VIEW_ITEM(bContext *C,
             return WM_UI_HANDLER_BREAK;
           }
 
+          if (ui_block_is_popup_any(but->block)) {
+            /* TODO(!147047): This should be handled in selection operator. */
+            force_activate_view_item_but(C, data->region, view_item_but, false);
+            return WM_UI_HANDLER_BREAK;
+          }
+
           if (UI_view_item_supports_drag(*view_item_but->view_item)) {
             button_activate_state(C, but, BUTTON_STATE_WAIT_DRAG);
             data->dragstartx = event->xy[0];