HIDEHANDLER is a function, when it return t, posframe will be
hide when `post-command-hook' is executed, this function has a
plist argument:
(:posframe-buffer xxx
:posframe-parent-buffer xxx)
The builtin hidehandler functions are listed below:
1. `posframe-hidehandler-when-buffer-switch'
https://github.com/alexmurray/flycheck-posframe/blob/66b73ddb93b357fe9b849d2aa14d5cc9e89e9ffd/flycheck-posframe.el#L225
From posframe document: