Block Patterns: previews overflow the container in some themes

[enriquesanchez]

Describe the bug In some themes (so far only tested on 2019, 2017 and 2015), the pattern preview overflows the container

To reproduce Using a theme other than 2020 or the Starter theme:

1. Open the Block library ('Add block')
2. Click on 'Patterns'
3. Scroll down the list of patterns
4. See error

Expected behavior Pattern previews should stay contained inside block-editor-block-patterns-list__item.

Screenshots

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Editor version (please complete the following information):

• WordPress version: 5.4.2-alpha-47847
• Gutenberg plugin Version 8.5.0-rc.1

Desktop (please complete the following information):

• OS: macOS 10.15.5
• Firefox 77.0.1

[enriquesanchez]

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