The main sample panel includes a scroll bar when the sample exceeds the size of the windows. The new opentype panel should also include this feature.
Probably not a bug; just a limitation of the new OpenType Sample Panel.
The scrollbar is available if you switch to text mode (click the X₂ icon).
If you want to display OpenType features you may have to reduce the font size, enlarge the panel, or reduce the amount of text.
You can define your own sample text for the panel in Options, Text Samples. This is what I use to test my OpenType features.
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We will add this enhancement to the next upcoming update of MainType.