Currently, when a user has Visual Diffs enabled for historical diffs (beta feature), the JS changes the URL of all diff pages to include the current mode as diffmode parameter. This is even done for the initial loading, i.e. even when the user doesn't change diffmodes, the URL will still be changed.
When users share links to diffs, they usually just copy the whole URL. Since that URL includes the diffmode parameter, there mode will be forced on other users. While in some cases this is the desired behavior, in most cases users just want to share the diff, not the exact mode. This is a bit annoying for other users, to have their preferred diff mode changed (and not only for that diff, but for all diffs until the explicitly change it again) just because they followed a link.
The best solution of course would be if users copied only part of the URL, omitting the diffmode parameter if they just want to share a link and only include it if they want to share the exact mode, too, but that's probably not going to happen, so the Visual Diffs should change their behavior instead.
Either the URL should never be adapted. This way, users never will share the diffmode if they don't really want to. They could still add that parameter manually, if they want to link to the exact mode, but - this is the disadvantage - they have to know about the parameter to do so. On the other hand, many other parameters aren't easily discoverable as well, so this wouldn't be very bad in comparison.
Or the URL should only be adapted when the user changes the diffmode, but not for initial loading. This would make it unlikely that URLs with accidental diffmode parameters are shared, while it would be automatically included in most cases where a user wants to share the exact mode.