mouse-autoselect-window

mouse-autoselect-window is a variable defined in `C source code'.
Its value is

nil

Documentation:
Non-nil means autoselect window with mouse pointer.
If nil, do not autoselect windows.
A positive number means delay autoselection by that many seconds: a
window is autoselected only after the mouse has remained in that
window for the duration of the delay.
A negative number has a similar effect, but causes windows to be
autoselected only after the mouse has stopped moving. (Because of
the way Emacs compares mouse events, you will occasionally wait twice
that time before the window gets selected.)
Any other value means to autoselect window instantaneously when the
mouse pointer enters it.

Autoselection selects the minibuffer only if it is active, and never
unselects the minibuffer if it is active.

When customizing this variable make sure that the actual value of
`focus-follows-mouse' matches the behavior of your window manager.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.