fix(ew): Adapt main content to account for margins

[magdalenaorlowska]

Description

• subtract 2px for border width to prevent a container-level scrollbar from appearing alongside the content scroll

Related Issue

adobe/da-nx#520

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.