Careful, folks...
The M adapter shown on the pictures may not have a cam follower which would be required to detect the lens's current focus distance. Without that information, the camera's optical viewfinder would not be able to move the bright-frame for parallax compensation. Also, the camera would not be able to calculate the in-viewfinder DoF display.
Frankie (which is active both here and on the Fuji-X forum) has provided more insights on the issue. See the referenced thread in the Fuji-X forum.
Should Fuji not make such an intelligent M adapter, Frankie might.