When I press the button nothing is shown in the composition area and I get the following output:

QOpenGLContext::swapBuffers() called with non-exposed window, behavior is undefined
QOpenGLContext::swapBuffers() called with non-exposed window, behavior is undefined

along with lots of the following:

cur_cycle = 6, issue_cycle = -1, ready_cycle = 0
cur_cycle = 7, issue_cycle = -1, ready_cycle = 0
cur_cycle = 8, issue_cycle = -1, ready_cycle = 7
cur_cycle = 8, issue_cycle = -1, ready_cycle = 7
cur_cycle = 8, issue_cycle = -1, ready_cycle = 8
cur_cycle = 8, issue_cycle = -1, ready_cycle = 8
cur_cycle = 9, issue_cycle = -1, ready_cycle = 9
cur_cycle = 2, issue_cycle = -1, ready_cycle = 1
cur_cycle = 2, issue_cycle = -1, ready_cycle = 0
cur_cycle = 3, issue_cycle = -1, ready_cycle = 1
cur_cycle = 3, issue_cycle = -1, ready_cycle = 0
cur_cycle = 4, issue_cycle = -1, ready_cycle = 1
cur_cycle = 4, issue_cycle = -1, ready_cycle = 0
cur_cycle = 5, issue_cycle = -1, ready_cycle = 1