I was very fascinated with Atari 2600 coding in the early 2000s, though I was very bad at it at the time. But I was lurking on the mailing lists where research was actively happening and tricks were being discovered. AMA! Listen to my podcast!
The black line in the corner of your screenshot there is a basically-unavoidable artifact of how HMOVE works and is called the “HMOVE comb”. If you look at Atari screenshots and pay attention you’ll see them everywhere. Activision had a policy of hiding the effect by ensuring that the first 8 pixels of every scanline were always black by strobing HMOVE on every scanline.
It’s possible through serious deep black magick (as opposed to the normal everyday black magick required to program the 2600 at all) to perform an HMOVE in such a way that the comb does not appear, but not only is it difficult to pull off, I think it is potentially not portable to different revisions of the chip. If you want waaaaaay too much detail about why this happens, I believe “TIA Hardware Notes (A Small Opus on the TIA)” by Andrew Towers is the thing to read - he reverse-engineered the schematics to really dig into how everything worked and explained a bunch of weird edge case behaviours that were not well understood at the time.