The heuristics that Wine uses to decide whether or not to trim off the edges of windows and replace them with the platform-native window decorations are imperfect. The Mac driver, like the X11 driver, has a registry setting to turn off window decorations for situations like this. Unfortunately, the Mac driver's setting can't be controlled using winecfg. You have to set it using regedit:
If you need help editing the registry check out this guide.