When editing movies, Timeline screen became dark

Question:

The whole timeline screen became dark from a certain point of time while editing.

Answer:

This could happen when you applied Clipping Effect unintentionally.

Clipping cuts out and shows only a part of the screen and the rest is shown dark.
While editing, LoiLoScope does not make a screen to a complete dark but shades it for easy clipping, so the editor might not notice even when it was applied mistakenly.
If you find Clipping tab on your Edit screen, you can disable it by clicking [Clipping]->[Delete]->[OK].