Why does my window cleaning robot leave a thin line of dirt at the edges?

That edge strip is basically normal, unfortunatly. The cleaning pad sits a little inside the housing so it can't touch the glass right up against the frame, since the frame lip itself is what the robot's edge brushes and sensors use to know where to stop. On most units you're looking at an inch or two of untouched glass around the border, and on a big pane it's really noticeable because the middle looks so good by comparison. The fix most people land on is exactly what you guessed: pre-wipe the border by hand before you let the robot loose. Takes two minutes with a damp microfiber cloth and then you don't have to think about it again. I do the top edge and the sides, since the bottom usually catches a bit of runoff and cleans itself during the run. Starting in the middle and letting it work outward isn't hurting anything, so no change needed there. Placement matters more for making sure the safety cord or tether has slack and that the pad path covers the widest part of the glass first.

The damp pad thing is real though. By the third window the pad is usually more smearing than absorbing, and that shows up worse at the edges where the dirt is concentrated anyway. Swap in a fresh pad or re-wet it between windows, or at least refill the tank partway through. Dry-ish pads on a big pane also just don't pick up as much on the outer passes.

One other thing to check: some robots do a deliberate edge pass at the end of the cycle where they creep along the frame. If yours has that, make sure the run is finishing fully and not getting interrupted, because a cut-short cycle can leave the border looking worse than usual.

So no, you're not doing anything wrong. Pre-wipe the frame strip, keep the pad wetter, and the robot will handle the rest of the pane fine.

Agreed on all of this, the pre-wipe habit is what made me stop noticing the border at all. One thing I started doing that helped more than I expected: I keep a small spray bottle next to me and give the pad a few squirts of plain water between windows instead of running back to the sink. Takes ten seconds and the pad stays absorbent way longer. On really dirty panes I'll even lift the robot off for a quick pad rinse halfway thru. Sounds fussy but it beats smearing the last window.

I'd push back on the claim that the bottom edge cleans itself from runoff. Mine is the opposite, the bottom strip is always the worst part of the pane because everything the pad pushes down just pools there and dries into a dirty line. If anything I run the robot, then do the bottom six inches by hand last, not first. Pre-wiping the border before is fine but I found doing the bottom after works better since you can see exactly where the grime settled. Also on mine the edge pass at the end is useless, it just rides over the frame lip half the time instead of the glass, so I stopped waiting around for it.

This thread is helpful timing beacuse mine kept sliding sideways a bit on the relly wide pane, like it wandered toward one corner instead of covering the whole glass. The pad was fine and the glass wasn't especally dirty, so I don't get why. Does the surface have to be perfectly flat for them to drive straight, or could a tiny warp in the pane make it drift? Also mine leaves faint round swirl marks if I let the pad get too dry, which matches what people are saying here.

I gave up fighting the border strip and just split the job. Robot does the big middle area every couple weeks, and the edge gets a quick wipe whenever I'm doing the sills anyway. Sounds lazy but the whole job still takes way less time than doing the full pane by hand used to. One thing that made the hand part faster: I wrap the microfiber around a squeegee handle so I can reach the top strip without a step stool. Also check the pad for grit before each run, I found a tiny crumb embedded in mine once and it left a faint scratch trail on the glass for the rest of that pass.