Thank you all for all the responses.
It was not TPanel as such - more a way to find what unit should be added to uses.
Ctrl+click or [F1], doesn't function, if uses does not list the unit.
It is useful that ctrl+click it opens the defining source (I didn't know this "trick") - but it is no use, to locating a unit.
Manually going to help, never occurred to me....
(Probably should spend some time there)
But found it, once I figured how to look in the lcl.hcm - it's not default, and not obvious that there are several helpfiles.
Saved the link
https://lazarus-ccr.sourceforge.io/docs/lcl/index-8.html.
with FF search, it is fairly simple to find what one is looking for.
And placing components manually, of course also places the required unit in uses.
But I have the problem when creating my own controls, or programmatically adding components.
And that sometimes requires, manually looking up where things are defined...
And now I have the tools to do that, rather than using "trial and error" method.
Thank you!