X.A.UpdateFocus: Add focusUnderPointer

Some people like their mouse pointer to move when changing focus with
the keyboard, other people like their pointer to stay and focus to
follow. xmonad(-contrib) supports both preferences, but imperfectly:
The former requires using the XMonad.Actions.UpdatePointer contrib
module, the latter (focusFollowsMouse) only reacts to CrossingEvent; the
focus isn't updated after changing workspaces or layouts.

This adds an inverse of XMonad.Actions.UpdatePointer.updatePointer that
immediately updates the focus instead.

Fixes: https://github.com/xmonad/xmonad/issues/108

CHANGES.md

@@ -686,6 +686,13 @@
     - Fixed issue with keyboard/pointer staying grabbed when a blocking action
       like `runProcessWithInput` was invoked.
 
+  - `XMonad.Actions.UpdateFocus`
+
+    - Added `focusUnderPointer`, that updates the focus based on pointer
+      position, an inverse of `X.A.UpdatePointer`, which moves the mouse
+      pointer to match the focused window). Together these can be used to
+      ensure focus stays in sync with mouse.
+
 ## 0.16
 
 ### Breaking Changes

XMonad/Actions/UpdateFocus.hs

@@ -16,7 +16,8 @@ module XMonad.Actions.UpdateFocus (
     -- * Usage
     -- $usage
     focusOnMouseMove,
-    adjustEventInput
+    adjustEventInput,
+    focusUnderPointer,
 ) where
 
 import XMonad
@@ -57,3 +58,25 @@ adjustEventInput = withDisplay $ \dpy -> do
   io $ selectInput dpy rootw $  substructureRedirectMask .|. substructureNotifyMask
                                 .|. enterWindowMask .|. leaveWindowMask .|. structureNotifyMask
                                 .|. buttonPressMask .|. pointerMotionMask
+
+-- | Focus the window under the mouse pointer, unless we're currently changing
+-- focus with the mouse or dragging. This is the inverse to
+-- "XMonad.Actions.UpdatePointer": instead of moving the mouse pointer to
+-- match the focus, we change the focus to match the mouse pointer.
+--
+-- This is meant to be used together with
+-- 'XMonad.Actions.UpdatePointer.updatePointer' in individual key bindings.
+-- Bindings that change focus should invoke
+-- 'XMonad.Actions.UpdatePointer.updatePointer' at the end, bindings that
+-- switch workspaces or change layouts should call 'focusUnderPointer' at the
+-- end. Neither should go to 'logHook', as that would override the other.
+--
+-- This is more finicky to set up than 'focusOnMouseMove', but ensures that
+-- focus is updated immediately, without having to touch the mouse.
+focusUnderPointer :: X ()
+focusUnderPointer = whenX (not <$> (asks mouseFocused <||> gets (isJust . dragging))) $ do
+  dpy <- asks display
+  root <- asks theRoot
+  (_, _, w', _, _, _, _, _) <- io $ queryPointer dpy root
+  w <- gets (W.peek . windowset)
+  when (w' /= none && Just w' /= w) (focus w')

XMonad/Actions/UpdatePointer.hs

@@ -60,6 +60,11 @@ import Control.Arrow ((&&&), (***))
 -- | Update the pointer's location to the currently focused
 -- window or empty screen unless it's already there, or unless the user was changing
 -- focus with the mouse
+--
+-- See also 'XMonad.Actions.UpdateFocus.focusUnderPointer' for an inverse
+-- operation that updates the focus instead. The two can be combined in a
+-- single config if neither goes into 'logHook' but are invoked explicitly in
+-- individual key bindings.
 updatePointer :: (Rational, Rational) -> (Rational, Rational) -> X ()
 updatePointer refPos ratio = do
   ws <- gets windowset