Pick one Pokemon in each slot and the comparison renders as soon as both are filled. You can swap either one at any time without losing your scroll position.
Step 1
On the compare page, click the first slot and type a name or dex number.
Step 2
Pick the Pokemon from the dropdown.
Step 3
Do the same for the second slot.
Step 4
The page updates with the full analysis. To swap either Pokemon, click the Clear button on its slot and pick a new one.