Increase specificity when applying properties inside Docs #26
Description
We inject the custom properties into #storybook-preview-iframe, which works for the vast majority of use-cases. However this means the properties won't be applied to the correct frame when Docs mode is running with iframe stories.
This also means there isn't isolation against the main story in docs mode.
Look to apply parameters more precisely, and when iframes are present apply them inside.
(Related: #25)