Question regarding virtual keyboard logic

[NavidZ]

So with built in widgets like input or content-editable the user agent knows that for example user is taping on that area so it kind of knows when for example to open a virtual keyboard. Obviously when VK is opened user agent is going to use the editContext buffers and cursor position in there to suggest stuff on VK and what not.
But I'd like to focus on showing and hiding virtual keyboard within EditContext. What should be now the heuristic for hiding and showing the VK so that it somehow gives control to the app as well.?