Oh i found the tricks ! but it's a little weird... if you not activate Staff Online in Members online widget it doesn't appears in ACP too...
Cannot reproduce this. It should show all of the time on the admin index as long as it finds users to display there.
I would still say that this is a "bug" in that if there is no title, it should center. If not, it's definitely a suggestion for a better appearing widget if some choose not to have a title while others do.
I don't think we generally agree with this. Note it's not unique to the admin CP though perhaps it looks worse there because it fits in a much wider area.
It applies to the front end widget too:
You may note if you look closely around the software that this style of an image with a title is used throughout the software. It's a core feature and is reused in many places.
We call it the "content row" system. By default, the title is aligned with the top of the row. We do have a variant that aligns things in the middle, but it isn't really consistent with everything else.
On the whole, having it aligned to the top generally looks better. As an example, if we employed
contentRow--alignMiddle
in places like the New posts widget, it would look like this:
Instead of:
Which, and maybe it's subjective, but IMO just doesn't look as good. So really on the whole this is as designed and is just the default behaviour of content rows that I don't think there's a compelling reason to change at this point.