What happens if you create a small simple database from scratch for testing purposes? Do you get the same issues?
If the issues are unique to a specific file, then that file may be damaged. You can try recovering the file and see if the recovered copy (test it even if no problems are reported by the recover), has the same problems.
If a brand new file shows the same issues, the problem would seem to be due to issues with your computer or your FileMaker app--most basic simple fix to try in that case is trashing your Filemaker preferences file. Beyond that, you might try testing your file on different computer to see if you get the same issues. This could be as simple as a keyboard that is no longer working correctly.
Thanks for the reply, PhilModJunk. I didn't follow up earlier because I didn't really have anything significant to report, other than creating a new database from scratch exhibits the same issues (thanks for that suggestion). I'm meddling with FMP layouts again today and encountered the problem again, so have looked a little more.
What I didn't mention is that I'm running FMP on a MacBook Pro (13-inch, Mid 2009), which I've upgraded with an SSD.
I managed to make this issue go away just now by putting the machine to sleep (via the Apple menu), then waking it again. !!
So I think is probably a hardware issue with this computer, or a fairly low-level bug in 10.8 (I'm now running 10.8.3), which I can (only) see manifested in FMP.
Any thoughts appreciated. I'd particularly like to hear from anyone that has seen the same issues (issues with closing drop-down menus, copying with opt-drag, object grids). But at least I have a work around now which doesn't involve logging out and in again (which I try and avoid because it takes me a while to get everything back to how I like it...)