Panel isn't visible when its size is exceeding the viewport height

[weaintplastic]

Describe the bug
We are using the controls add-on. When we have a large number of controls rendered, the panel isn't shown anymore as soon as it runs out of vertical space. We only see the preview and the panel is hidden behind it but not visible due to a low (non-existing) z-index.

storybook/lib/ui/src/components/layout/container.tsx

Line 139 in 1e97c26

export const Panel: FunctionComponent<{

To Reproduce
I found only one consistent way to reproduce the bug.

1. Increase the Panels height by dragging it to the top
2. Reduce the browsers window and make the Panel "overflow" the Preview
3. Reload the browser
4. Wonder where the Panel is gone
5. Increase the height of the browser window
6. See your panel again

Expected behavior
Have the panel always visible.

Screenshots
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Code snippets
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Additional Information

Why is this a problem? I guess the height of the panel is stored somewhere in local storage. Since we are embedding storybook into a styleguide, the available viewport there is much smaller than the browsers window. If someone is visiting our storybook application and dragging the panel (height stored in local storage) and is then visiting the styleguide where the same story is loading in an iframe, the panel's height is too big and therefore not rendered in the styleguide.

At least that's what I imagine what's happening.

[weaintplastic]

As a hotfix I'm deleting the local storage entry in .storybook/preview.js

localStorage.removeItem('storybook-layout');