This is to work around using Select.Trigger as a composite item by letting CompositeItem's tabIndex override the internal one in useButton

Like Menu.Trigger, select trigger manually sets tabIndex to keep a consistent "trigger remains focused after a select/menu item is select" behavior across browsers (Safari on iPadOS and iOS): https://github.com/mui/base-ui/blob/master/packages/react/src/select/trigger/useSelectTrigger.ts#L80

I wonder if this could cause problems with buttons nested inside other components within a toolbar (for example, a button inside a dialog opened from a toolbar would have this set to true, even though it's not a composite item).

for example, a button inside a dialog opened from a toolbar

Only if that button happened to use useButton as well, so in this case, a disabled Dialog.Close isn't affected, but a MenuTrigger in that dialog would be

What this affects is that when disabled, the HTML disabled attribute is never applied. Generally I think our components that use useButton already don't rely on the disabled attr to disable functionality anyway in order to support component/element replacement so it should be OK in most cases

@michaldudak I also tried making it a isCompositeItem param instead of a check but it wasn't enough to handle this more common case: 575dc96