Improve smooth scroll

[Spindlyskit]

Currently, smooth scroll is considerably slower than default scrolling when scrollline is called during an active scroll, it seems this is because the new endpoint is based of the current scroll position rather than the current destination. This change calculates the new endpoint using the previous endpoint rather than current position if a scroll animation is already in progress.

[glacambre]

Hi, thanks for opening this PR! The code looks good to me but scrolling is super tricky to get right so I'd like to test it on these websites before it is merged:

• https://en.wikibooks.org/wiki/Ada_Programming/Generics (Smooth scrolling is terrible #627)
• https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html (Smooth scrolling is terrible #627)
• https://kanze.co/blog/cinematography-in-user-experience-design (Contribute some stuff upstream? #826)
• https://riot.im/app/#/room/#tridactyl:matrix.org (Trying to scroll up repeatedly in chat history on Riot crashes the tab #1247)
• https://stripe.com/docs/api (scrolling.ts: Fix sticky scrolling #1143)
• https://www.viva64.com/en/b/0574/ (Trying to scroll beyond the end of a page sometimes makes scrolling up slower #777)
• https://bugzilla.mozilla.org/show_bug.cgi?id=1456485 (Scrolling is broken on BMO #762)
• https://jonas.github.io/tig/doc/manual.html (j/k scroll doesn't work on "checkout" modal on Humble Store #63)
• https://twitter.com/bbcpress/status/931117259791241216 (j/k scroll doesn't work on "checkout" modal on Humble Store #63)
• https://humblebundle.com/ (j/k scroll doesn't work on "checkout" modal on Humble Store #63)
• https://twitch.com/ (Scrolling doesn't work on Twitch #218)
• http://www.angelfire.com/super/badwebs/ (No interaction on websites with frames #122)
• https://pinafore.social/ (j/k scrolling broken on pinafore.social #372)
• https://gatesnotes.com/ (Scrolling doesn't work on gatesnotes.com #273)
• http://just-blinds.co.uk/blinds-manchester/ (Support 'n' for Find Again #64)
• http://docs.nvidia.com/cuda/parallel-thread-execution/index.html (Scrolling is slow on pages with lots of elements #395)
• http://diana-adrianne.com/purecss-francine/ (Scrolling is slow on pages with lots of elements #395)

I'll do that in a few hours :).

[Spindlyskit]

I've briefly looked over the sites (I don't exactly know what tests need to be done) and almost all seem to work correctly, that said, I was unable to get the riot room to scroll however this was also the case on the latest tridactyl from amo with smoothscroll on or off, this is likely a problem on my end.

The other issue I had was with twitter, I found it would scroll as expected on both the latest (amo) and my changed version of tridactyl whilst logged in however the page would not scroll without a login session. This issue also seems to be present on latest tridactyl and might need investigation, it seems to be caused by the logged-out version of twitter rendering the profile feed in the background of the post which does not happen when logged in.

EDIT: The twitter issue seems to be (#1521)

[glacambre]

Thanks for taking the time to test your PR on these websites. I tested it too (the test is just checking if scrolling still works and didn't become super slow both with and without smoothscroll - we've had seemingly innocuous changes break scrolling in bizarre ways before) and everything works as expected.

[glacambre]

Makes smoothscroll much better - to the point where we can probably close all issues about smoothscrolling. Thank you!