Window focus follows mouse

Is your feature request about something that is currently impossible or hard to do? Please describe the problem.
iterm2 and various terminals have a feature where the window focus can follow where the mouse pointer is above

Describe the solution you'd like
if I have multiple macvim windows open and I move my mouse over a different window, that window should have the keyboard input focus

首先定位 MacVim 在 macOS 上处理窗口焦点的代码，以及鼠标移动如何在多个窗口之间传递。打开多个 MacVim 窗口来验证行为：将指针移动到某个窗口上时，该窗口应获得键盘焦点。