Scissor Test And Framebuffer Clearing
From OpenGL Wiki
Why doesn't glClear() work for areas outside the scissor rectangle?
The OpenGL Specification states that glClear() only clears the scissor rectangle when the scissor test is enabled. If you want to clear the entire window, use the code:
glDisable (GL_SCISSOR_TEST); glClear (...); glEnable (GL_SCISSOR_TEST);