can't edit drafts in mail

Using new iMac and big sur but cannot edit drafts in mail, for example if i try to delete one letter, the whole draft document erases or if I try to add text nothing happens. Everything works fine in for example notes, everything works fine on my ipad and iPhone in mail but not on the iMac

I have used first aid removed the gmail acct to no avail. When I go via a browser to my gmail account everything works well.

Been on with apple support between the phone agent and chat for together 4 hours but even though they tried an herculean attempt, nothing works. Ideas welcome.

gj