Start a selection, drag the handle at the top (outside of the view). The document should scroll, allowing to create selection larger than what is visible on the screen. [Related: Currently when you try to move the srollbar in the middle of the selection, it does strange things. Please feel free to file as a separate bug if unrelated.]
Andrzej Hunt committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/online/commit/?id=acb39fc057db157b988f2cabf50809ab7a582dcd fdo#94610 autoscroll document for selection
Fixed - thank you!