Add function to detect floating windows in ManageHook

Add a willFloat function to deteect whether the managed window will be
floating or not

Add description of added change to CHANGES.md

CHANGES.md

@@ -51,6 +51,9 @@
   * Added `withUnfocused` function to `XMonad.Operations`, allowing for `X`
     operations to be applied to unfocused windows.
 
+  * Added `willFloat` function to `XMonad.ManageHooks` to detect whether the
+    (about to be) managed window will be a floating window or not
+
 [these build scripts]: https://github.com/xmonad/xmonad-testing/tree/master/build-scripts
 
 ### Bug Fixes

src/XMonad/ManageHook.hs

@@ -106,6 +106,14 @@ getStringProperty d w p = do
   md <- io $ getWindowProperty8 d a w
   return $ fmap (map (toEnum . fromIntegral)) md
 
+-- | Return whether the window will be a floating window or not
+willFloat :: Query Bool
+willFloat = ask >>= \w -> liftX $ withDisplay $ \d -> do
+  sh <- io $ getWMNormalHints d w
+  let isFixedSize = isJust (sh_min_size sh) && sh_min_size sh == sh_max_size sh
+  isTransient <- isJust <$> io (getTransientForHint d w)
+  return (isFixedSize || isTransient)
+
 -- | Modify the 'WindowSet' with a pure function.
 doF :: (s -> s) -> Query (Endo s)
 doF = return . Endo