There is a bug where if you have the game fullscreen and scroll your scroll wheel, there is a chance your selection will go crazy. But if you leave fullscreen, it doesn't bug out. *(I couldn't attach the files here, so here's two links. One for each video.) Fullscreen: https://send.firefox.com/download/d070727af0746d1c/#A8IJwD7yFynato9p9jJ13A
Windowed: https://send.firefox.com/download/c2786c6f0fdb4098/#hcPi50l2hxk7HlAhwOa_Hg
These links expire in 7 days.
Thank you for your report!
We're actually already tracking this issue at MCPE-29424, so I will resolve and link this ticket as a duplicate.
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