[BUG] Large Off-Canvas Header Bar Missing Z-Index

[Xenopotamus]

Describe the bug
The large off-canvas has a header bar that is missing the proper z-index. When the user scrolls, the header bar's shadow does NOT appear on top of content below it.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'https://tangoe.design/ui-kit/off-canvas'
2. Make the height of your browser window very small so that you can scroll vertically.
3. Scroll down and note how the shadow of the header bar does not appear on top of the content below it.

Expected behavior
The header bar's shadow must remain on top of the content within the large off-canvas.

Screenshots
Issue:

Expected (after adding z-index):