Just to be clear:
Toggling "High Contrast Mode" on and off does not fix this.
The issue happens regardless of this setting.

As of Steam Version: 1752534859 :

"High Contrast Mode" disabled:
Only the tray context menu is still affected, the other cases seem to be fixed.

"High Contrast Mode" enabled:
All reported affected places (webpage context menu, notifications panel dropdown, tray context menu) are still affected by this visual bug.

Thanks for the report, this will be fixed in the next client beta.

The issue is back in Steam Version: 1753138871.
Seems whatever you did to fix the issue with the expanding menus made this one return more pronounced.

More details:
- the black line on the right border of the tray menu is back (both in normal mode and high-contrast mode)
- Notification Panel: Toggling High-Contrast on and off caused a grey border at the right side to appear (until client restart)
- In High-Contrast Mode there's a very thin (1px) black line on the webpage context menu, notification panel, "Manage account" dropdown as well as the "Steam", "View" and "Friends" dropdown menu (but seemingly not on the "Games" and "Help" menu!)

EDIT: All of this has been tested with default UI Scale (set to 1).

These issues still exist in Steam Version 1753318274:

- High Contrast Mode: thin black line on the right side of certain UI elements (outside the white border around the item): Notification Panel, website context menu, several menus (Steam, View, Friends, Manage account)

- Normal Mode: Toggling High Contrast Mode on and off again will cause a grey border to appear at the right side of the notification panel (persists until client restart)

Also, in normal mode:
the tray menu still shows the black line for a split second after opening.
This only happens on the first appearance of the tray menu after a client restart.
Please fix these issues.