When you apply for funds from a research council for a postdoc position, and the potential host is a university/organization that you don't know, do you share your research proposal with the PI of that university/organization beforehand?
I want to avoid plagiarism, so I'm planning to have a quick chat with the PI only, or simply apply for funds first, and discuss with the PI later if I get the grant.
If you don't share the proposal with the prospective host, you are missing out on the chance of getting feedback from them. Detailed expert feedback on a grant application is not easy to get, and very useful.
If I would distrust a colleague so much that fear of them stealing my ideas outweights this potential benefit, I'd be very, very reluctant to consider them as a postdoc mentor.
All the calls for ad hominem postdoc fellowships that I've seen require, as part of the application pack, a letter of support from a senior person at the proposed host institution, and a copy of the host institution finance team's detailed estimates of the "indirect costs" associated with carrying out the project at that institution. I can't see how that could be done without people at the host institution seeing the proposal.
Seconding the answer by Arno, I'd like to add that if I'm an academic staff member at the host institution, I want to know in detail what a postdoc is going to do in "my" group before any non-reversible actions are taken. To the very least, I want our research interests to be aligned (to some extent), and to verify this, I'd absolutely need to read a proposal of the applicant.
If I learn that the applicant does not trust me enough to show me their proposal, that would be a clear no.