The controls disappear when clicking through different tasks. i.e. changing to multi-picture library view from single picture or library to develop.
below is a before and after screenshot. The controls reappear on when the screen is hidden and then recovered but returns to blank as soon as I change pages.
It is a known issue although usually intermittent on some Mac OS computers. You could follow the thread below for progress. A couple of work-arounds:
Press G to go back to the Library grid followed by D to get back to Develop.
Minimize your window and then click the LR dock icon to re-open.