Block Styles panel flashes in IE11

[kjellr]

Describe the bug

In IE11, moving your mouse over the block styles panel triggers a flash of a clear, scrollable area:

To Reproduce

1. Use IE11.
2. Insert a block that includes Block Styles (Separator or Quote).
3. Click the block chip on the block toolbar, to bring up the style options.
4. Move your mouse over the style options.

If the panel expands up, you'll see something like these GIFs:

If the panel expands down, you'll see something like this:

Tested in
Windows 7, IE 11.0.9600.19080
Gutenberg 3.4