After the overwrite button, the timeline remains in the black frame. Previously, when I transferred a video to the timeline, it stayed on the video frame. now it stays on black screen and i can't work comfortably.
As far as I can tell, what you are seeing is normal. I am not sure why you would see a video frame if you assembled a sequence of clips one after another.
If you see a frame of video and then overwrite, you would be cutting the last frame of the video clip you are viewing which is incorrect.
To unlock this mystery, zoom in to the playhead to see that the playhead has both a head and a tail. It should make sense once you closely inspect it. To the right of the tail is that black frame where you will place your next clip or fade to black. I hope that makes sense.
That's not really the case, because the playhead is going to be ready for you to insert the next video clip. So after you've done your insert/overwrite shortcut the playhead is sitting at the first blank frame after the clip you've just added to the timeline. Otherwise when you insert/overwrite the next clip you'd be chopping the last frame off your clip.