Unfortunately this is a known bug in xmonad-contrib v0.12:
https://github.com/xmonad/xmonad/issues/15
If you are building from source just use git master.
We really need to make another release.
Jan Christoph Uhde
I experience the same issues as in:
https://mail.haskell.org/pipermail/xmonad/2016-March/015047.html
avoidStruts works for me in 0.11.1 (not compatible with the newest ghc 8.x.y in debian sid). In version 0.12 (cabal-default) it does not work on the first workspace, but starts working after pressing (mod-shift-space). In current git version xmonad (8b055621e92e7ade127043e968f50713c15a00a0) / xmonad-contrib (ec5f9a9e59ed5fa6bbd4c3850e4025526155f86e) my xmobar is never visible as soon as there is any other window.
I have tried to work with sanboxes and different ghc versions but it is all very painful as I have to cd into the correct sandbox and even then xmonad must use a certain ghc version on startup when it compiles it's configuration.
Please let me know how I can help debugging the problem. I know a bit programming and a few haskell basics. So please give me enough details to be helpful. You will find me as obiwahn in #xmonad.
-- Peter Jones, Founder, Devalot.com Defending the honor of good code