the tooltip is misplaced if page or component is vertically scrolled

[watery]

I noticed this with the vanilla js component integrated in a wiki page.
And it can be seen in this codesandbox: https://codesandbox.io/s/serene-poincare-35wsc8 (copied from #7) - even if it's using the React integration - where the gantt component has a fixed height of 1000 pixels. Just scroll the chart a little and then hover any activity: the tooltip is not placed beneath the blue box:

I found that changing the CSS .gantt-tooltip { position } property from fixed to absolute fixes the problem, but was just a quick attempt that surely needs testing: