There are a lot of addons that imply a permanently open sidebar - Tree Style Tab (177k users), Simple Tab Groups (64k users), Sidebery (34k users), Tab Stash (21k users) and a bunch of others that are not related to tabs. In your addon, the "fit-to-window" image scaling mode still doesn't take into account the open sidebar. Your addon scales the image really "fit-to-window", but should scale them "fit-to-content-area-visible".
For all these hundreds of thousands of users, your "fit-to-window" mode is meaningless and useless.