After
Right click → Item Comparison
the Item Comparison window commandeers the focus of the window that held the item you're comparing. This has inconsistent effects, as you can compare items from several places.
If you select Item Comparison from the chat log, the item comparison window appears over your chat log. If you then slash or enter to focus the input box, the item comparison closes. So if you want to discuss a linked item that you're comparing as you're comparing it, you have to repeatedly open the Item Comparison window. Over. and over.
When selecting Item Comparison from the armoury chest or your inventory, the Item Comparison window appears over whichever interface holds the comparison item, and that particular item container becomes unresponsive until you close the comparison window. If you're debating over two or more possible upgrades, you have to
Right click item A → Item Comparison → Close → Right click item B → Item Comparison → Close → Right click item A → Item Comparison → Close → Right click item B → Item Comparison → Close → Right click item A → Item Comparison → Close → Right click item B → Item Comparison → Close → Right click item A → Item Comparison → Close → Right click item B → Item Comparison → Close
Adding an automatic comparison to the Item Help tooltip would have the effects of creating consistency in the functionality of item comparison on several fronts, as well as adding a level of convenience.