Fix page up/down scrolling only half a page

[aaronraimist]

Fixes element-hq/element-web#3763

[grahamperrin]

0.9 of a page feels/looks good to me.

---

As far as I can tell, the only oddity, and this is out of scope, is when not enough of the next page is 'buffered' before paging down. When, for example, there's relatively little space between the foot of the scroller and the foot of the scrollbar.

Like, the lowest line of text becomes uppermost (or near uppermost) ☑ momentarily, then a bounce back of around half a page, leaving the line near mid-screen.

This was not a frequent occurrence. It might occur only when speed-reading. I did not attempt to make it reproducible, but if I recall correctly: when the ~half-page bounce occurs, the next Page Down will 'revert' the bounce (i.e. ~half a page down) then subsequent strokes of Page Down will behave normally (0.9).

[t3chguy]

See topic of #element-web:matrix.org

[grahamperrin]

/me thanks @t3chguy and switches from the deploy-preview server to https://develop.element.io/