Layout modifier for user provided per-window aspect ratios.

You can use this module with the following in your xmonad.hs:

import XMonad.Layout.FixedAspectRatio

Then add it to your layout:

myLayout = fixedAspectRatio (0.5, 0.5) $ Tall 1 (3/100) (1/2)  ||| Full ||| etc..
main = xmonad def { layoutHook = myLayout }

Which will center the (eventually) shrinked windows in their assigned rectangle.

For a layout modifier that automatically sets the aspect ratio depending on the size hints (for example for programs like mpv), see XMonad.Layout.LayoutHints

See the tutorial and XMonad.Doc.Extending for more info on the layoutHook.

You also want to add keybindings to set and clear the aspect ratio:

     -- Set the aspect ratio of the focused window to 16:9
  ,((modm, xK_a), withFocused $ sendMessage . FixRatio (16 / 9))

     -- Clear the aspect ratio from the focused window
  ,((modm .|. shiftMask, xK_a), withFocused $ sendMessage . ResetRatio)

There's one caveat: to keep the usage of the modifier simple, it doesn't remove a window from its cache automatically. Which means that if you close a program window that has some fixed aspect ratios and relaunch it, sometimes it'll still have the fixed aspect ratio. You can try to avoid this by changing they keybinding used to kill the window:

 , ((modMask .|. shiftMask, xK_c), withFocused (sendMessage . ResetRatio) >> kill)

See the tutorial for more info on customizing the keybindings.

This layout also comes with a ManageHook doFixAspect to automatically fix the aspect ratio:

myManageHook = composeOne [
  title =? "Netflix" <||> className =? "vlc" --> doFixAspect (16 / 9)
  ...
]

Check the tutorial and XMonad.Doc.Extending for more information on customizing the manage hook.