Fix QScroller overshoot at the bottom of chat history.

[gearonixx]

Scrolling at the bottom of a
fully loaded chat doesn't stop at the last message.

Most visibly with the experimental QScroller option enabled.

Verified on a Debug build against latest dev.

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[ilya-fedin]

Can you separate commits into standalone PRs and sign the CLA? Your first commit completely breaks overscroll in chat history currently.