Playhead cursor has a ghostly trail, won't cut precisely

Forgive me if this has come up before, I'm struggling to describe it so I'm not having any luck with searching previous questions. Recently on 10.4.9, when I move my cursor over the timeline, the red straight line gets a shadow. Then when I click to make a cut, the cut can appear where the line is, but more often at the edge of this shadow. Is this a setting that I've accidentally triggered? It's bewildering me and I'd be so grateful for any advice on how to switch it off. Here's a screengrab which might be easier to understand than my ropey description.

You are zoomed in too close. That light area is the actual frame. Your playhead is in scrub mode. Zoomed in that close, you can scrub at a sub-frame level for audio editing.

Video is frames per second.
Audio is samples per second, normally 48,000 samples per second.

You can only make video cuts on a frame, not on an audio sample level.

The Command key with the plus and minus keys zoom you in and out of your timeline.
Shift-Z will do a fit-to-window for your timeline.

Zoom out a bit.

Superb, thank you very much. I know I zoomed in for that shot, but I hadn't realised that I'd zoomed in at all on my actual work and yet clearly I have. Thanks, hugely appreciated.

Remember that a frame does take up some duration of time. 24fps a frame is 1/24 of a second, 60fps is 1/60 of a second. Zooming in that close, you can see that duration of time for the frame.

Thanks,