3.0: Grouped Piece Types in Expand to Fill Width of Dropdown

PLEASE NOTE: make sure the bug exists in the latest patch level of your major version of Apostrophe. For instance, if you are running a 2.x version of Apostrophe, you should use the latest in that major version to confirm the bug.

## To Reproduce

Step by step instructions to reproduce the behavior:

> 1. Add Grouped Piece Types to the menu bar
> 2. Click on a group heading (especially if a piece type has a longer name)
> 3. See error

## Expected behavior
> Other top level menu items should not move

## Describe the bug
> Other items move to accommodate the dropdown

## Details

**Version of Node.js:**
*PLEASE NOTE: Only stable LTS versions (10.x and 12.x) are fully supported but we will do our best with newer versions.*
v15.2.0 (although bug is in frontend)

**Server Operating System:**
*The server (which might be your dev laptop) on which Apostrophe is running. Linux? MacOS X? Windows? Is Docker involved?*
macOS Big Sur. I am using Docker to host MongoDB, but I don't think that's a contributing factor.

**Additional context:**
> Add any other context about the problem here. If the problem is specific to a browser, OS or mobile device, specify which.
Only tested on Safari

**Screenshots**
*If applicable, add screenshots to help explain your problem.*

Start by reproducing the grouped piece types dropdown described in the issue, especially with a longer piece type name, in Safari. Compare the menu before and after clicking a group heading; done means the dropdown opens without moving other top-level menu items.

Written by the indexing model from the issue text.