git-gui, BUG: minimize/maximize buttons missing when repo is opened in gui

System: Ubuntu 17.10 Gnome

When opening git-gui from a directory which is a repository, minimize
and maximize buttons are showing and functional in git-gui.

However, if I open git-gui in a non-repo directory, git-gui opens a
dialog where I can "Create New Repository", "Clone Existing
Repository", etc. When a repository is opened from this dialog, the
git-gui window does not have minimize and maximize buttons visible.

Steps to reproduce:
1. Open git-gui in a non-repository directory
2. Click "Open Existing Repository"
3. Click "Browse" and select an existing repository directory
4. The repository is opened with git-gui, but without
minimize/maximize buttons visible