Horizontal line appears while toggling vertical tabs sidebar in fullscreen

1. Disable the "Always show toolbar in fullscreen" option
2. Create a second tab in the window with ⌘ + N.
3. Focus away from the toolbar so that it is hidden.
4. Open the vertical tabs sidebar with the shortcut.
5. Change your tab by clicking on the vertical tabs sidebar
6. Focus on the page so the toolbar is re-hidden.
7. Close the vertical tabs sidebar with the shortcut.
8. Notice the horizontal line that has appeared.

There should be no horizontal line visible.

Version 0.99.129.4-rc (WebKit 621.1.2.111.4)

Sequoia (15)