Constrain popup dialog height in JBrowse (#300)

* Constrain popup dialog height in jbrowse
* Fix vertical height calculation for correct overflow

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Co-authored-by: Sebastian Benjamin <sebastiancbenjamin@gmail.com>

Author: bbimber

jbrowse/src/client/JBrowse/Browser/plugins/ExtendedVariantPlugin/ExtendedVariantWidget/ExtendedVariantWidget.tsx

@@ -407,7 +407,7 @@ export default jbrowse => {
         feat["INFO"] = null
 
         return (
-                <Paper data-testid="extended-variant-widget">
+                <Paper data-testid="extended-variant-widget" style={{height: '100vh', overflowY: 'scroll'}}>
                     {message}
                     <FeatureDetails
                             feature={feat}