Can't figure out how to place widget at the bottom of editor

I’m trying to place some note summary widget to be used with Obsidian, but can’t place it to the bottom of edtior correctly. I’ve placed it as RangeSetBuilder and set document size as from/to positions. But when i type text, it goes below widget for some time. Then everything renders again and widget goes below the typed text.
I want to keep widget at the bottom whole time.