keep overflow, prevent events that cause scrolling

[valdrinkoshi]

(started in #64, which was reverted because it introduced a breaking change)

Fixes #54 , fixes #43.

For #54, the goal is to keep the overflow on document.body, and prevent the scrolling from happening if:

• the scroll happens outside the locked element
• the scroll happens inside the locked element, but the target is contained in scrollable parents that cannot scroll (because they're at their boundaries already)

For #43, the scroll event listener on document will restore scrolling if it happens because of a user text selection or if the user scrolls through the side scroll bars. If allowScrollOutside, it will call refit.

I've profiled the performance of the change using this file https://gist.github.com/valdrinkoshi/5fb1c67fddd31782508b9e39c8ac25a3, and I'm seeing that overall fps is around 60fps, except some longer frames which go down to 25-30fps, and I believe it is mainly because of the touchmove listener which interrupts the scrolling to execute the javascript (in the timeline I see only the Painting and Rendering activities during this longer frames).

Interactions:

Longer frame example:

[valdrinkoshi]

FYI for #54, I came up with another way to solve it in #95

[e111077]

Does this check the composed tree as well? 🌴

[valdrinkoshi]

Actually, let me revert this part as it has nothing to do with the fix ;)

[valdrinkoshi]

Done!

[e111077]

Testing on nexus 6p and pixel c; CPU usage while scrolling on unscrollable section is very low. Two cores used, and mostly less than half. No frame drops; all idle frames!!

Slower when scrolling in a scrollable section. CPU usage seems to be relatively high, but no higher than the animation of opening the dropdown. No frame drops except when flinging, but that happens on the main page too with no open dropdowns (damn android). I think that the memoization is a success.

[e111077]

LGTM

[e111077]

fixes #91