The XSetDeviceFocus request changes the focus of the specified
device and its last-focus-change time. It has no effect if the
specified time is earlier than the current last-focus-change
time or is later than the current X server time. Otherwise, the
last-focus-change time is set to the specified time CurrentTime
is replaced by the current X server time). XSetDeviceFocus
causes the X server to generate DeviceFocusIn and
DeviceFocusOut events.

Depending on the focus argument, the following occurs:
  * If focus is None , all device events are discarded until a
    new focus window is set, and the revert_to argument is
    ignored.
  * If focus is a window, it becomes the device's focus window.
    If a generated device event would normally be reported to
    this window or one of its inferiors, the event is reported
    as usual. Otherwise, the event is reported relative to the
    focus window.
  * If focus is PointerRoot, the focus window is dynamically
    taken to be the root window of whatever screen the pointer
    is on at each event from the specified device. In this
    case, the revert_to argument is ignored.
  * If focus is FollowKeyboard, the focus window is dynamically
    taken to be the window to which the X keyboard focus is set
    at each input event.

The specified focus window must be viewable at the time
XSetDeviceFocus is called, or a BadMatch error results. If the
focus window later becomes not viewable, the X server evaluates
the revert_to argument to determine the new focus window as
follows:
  * If revert_to is RevertToParent, the focus reverts to the
    parent (or the closest viewable ancestor), and the new
    revert_to value is taken to be RevertToNone.
  * If revert_to is RevertToPointerRoot,
    RevertToFollowKeyboard, or RevertToNone, the focus reverts
    to PointerRoot, FollowKeyboard, or None, respectively.

When the focus reverts, the X server generates DeviceFocusIn
and DeviceFocusOut events, but the last-focus-change time is
not affected.

Input extension devices are not required to support the ability
to be focused. Attempting to set the focus of a device that
does not support this request will result in a BadMatch error.
Whether or not given device can support this request can be
determined by the information returned by XOpenDevice. For
those devices that support focus, XOpenDevice will return an
XInputClassInfo structure with the input_class field equal to
the constant FocusClass (defined in the file XI.h).

XSetDeviceFocus can generate BadDevice, BadMatch, BadValue, and
BadWindow errors.

The XGetDeviceFocus request returns the focus window and the
current focus state.

Not all input extension devices can be focused. Attempting to
query the focus state of a device that can't be focused results
in a BadMatch error. A device that can be focused returns
information for input Class Focus when an XOpenDevice request
is made.

XGetDeviceFocus can generate BadDevice, and BadMatch errors.

BadDevice
       An invalid device was specified. The specified device
       does not exist or has not been opened by this client via
       XOpenInputDevice. This error may also occur if the
       specified device is the X keyboard or X pointer device.

BadValue
       Some numeric value falls outside the range of values
       accepted by the request. Unless a specific range is
       specified for an argument, the full range defined by the
       argument's type is accepted. Any argument defined as a
       set of alternatives can generate this error.

BadWindow
       A value for a Window argument does not name a defined
       Window.

BadMatch
       This error may occur if an XGetDeviceFocus or
       XSetDeviceFocus request was made specifying a device
       that the server implementation does not allow to be
       focused.