It's not your computer, it was me running out of time to polish it. The highlight is a fixed width, and the collision areas for the items are small to avoid a bug I didn't have time to fix where you couldn't select one item if it was close to another. It only looks okay in the GIF because I selected items with short names and I've played it a hundred times and am expert at selecting the items.😅
For your question about luck, I think you're asking about the gameplay itself, correct me if I'm wrong. Each item has a base damage which is the same every time, and then each playthrough items can get a special quality (e.g. formerly French, soaked in holy water, made of glass) which raise their damage or might cause them to damage you.
Thanks for your comment!