Always eat click events if they're inside your own object, even if you don't do anything with them.
_isinobject() "lies" if click happens in hidden part of half visible object inside virtual group. So for things to work the click must be filtered out elsewhere by MUI itself.
Maybe it's like this: Old MUI used to always check if filtering is required. Then someone invented MUI_EHF_GUIMODE and then changed click-kill-filtering to be done only for event handlers with MUI_EHF_GUIMODE flag. And all internal MUI classes were changed to use that flag anyway. So nothing changed for those. And for normal event handlers without MUI_EHF_GUIMODE the checks/filtering was not done anymore. And this breaks stuff which does not use or know about MUI_EHF_GUIMODE.