Split view "options" button shows square artifact behind it

[Brave-Matt]

Description

When you enable Split View in Brave (Nightly, Beta and Stable w/flag), the overflow/"more options"/overflow button on the separator shows a square artifact behind it, overlapping both tabs in the view:

Note that the artifact is more prominent in dark theme, given that the background of the square is always light, but still very noticeable in light theme as well.

Steps to reproduce

1. Enable Split View
2. Observe square artifact behind split view separator menu

Actual result

Artifact is shown

Expected result

No artifact should display

Reproduces how often

Easily reproduced

Brave version (brave://version info)

v1.76.74 and higher

Channel information

• release (stable)
• beta
• nightly

Reproducibility

• with Brave Shields disabled
• with Brave Rewards disabled
• in the latest version of Chrome

Miscellaneous information

cc @rebron