Fix incorrect resizing of sidenav

[stefvanschie]

Proposed changes

This fixes issue #6350. When the sidenav is in non-fixed mode and opened, then the window is maximized making the window wider than it was previously, the darkening effect of the sidenav would stay. This has now been changed, so the darkening effect properly disappears.

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 read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.