When you press the minimize button and then return to Chromium from a minimized state you might encounter the issue described below.

Chromium is minimized.

When Chromium is brought back from the minimized state it might show up like this.

Reason: The Chromium window is returned to an unmaximized state and Fluxbox does not know how to handle it correctly with Chromium. The solution is simple: Press the maximize button and your window should show up correctly again.