Automatically capture cursor and keyboard on window focus

[acheronfail]

Description of the Feature

I'd like to have an option to automatically capture the cursor and keyboard whenever my VM window is activated (gains focus).

Right now, I have to remember each time to tap the modifier combination, and it's quite frustrating when constantly switching between the host and the VM.

Attempted workarounds

I've tried to script with this applescript/hammerspoon, but since the key combination is modifiers only, these applications can't seem to trigger it.

Suggestions

This could be solved in a few ways:

1. allow changing the key combination to trigger the capture mode, or at least offer another variant which includes a non-modifier key (this would allow other script apps like hammerspoon, etc to be able to hook into it)
2. add an option "auto-capture on VM window focus" which will automatically capture the cursor and mouse whenever the window is activated (to 'un-capture' the original key combination could be used)
3. simply add a menu item which "captures cursor and keyboard" (if there's a menu item, then some apps can read those and trigger them - System Preferences can do this, applescript, etc)

[ylluminate]

In conjunction with this, it would be nice to have some kind of an open source hook that could be utilized in Linux, FreeBSD and elsewhere to auto-release when moving outside of the VM-guest...