MacBook Pro 15"2018 - Glitches Safari on External Display Clamshell - macOS 10.14.1

[precision01]

Hello.

I've been having some problems for a couple of days whenever I use my MacBook Pro with external display in clamshell mode. I have some weird glitches when using Safari. It is strange that this only happens in safari, and when mouse cursor rolls over some webpage components, like the one on video. I had the glitches on some comboboxes and other elements too. See the video:

Some things I tested and noticed:

1. The issue only happens when using MacBook Pro in clamshell mode. When MacBook Pro lid is open there's no glitch.
2. Tried to change displayPort cable but the issue is still there.
3. Tried to connect external LG display to different thunderbolt ports but the issue remains there.
4. Sometimes, if the webpage has too many elements, and you keep moving mouse over those elements, the external display goes black for 1 second.
5. I couldn't reproduce the issue on Chrome or Firefox.

Is someone else having similar issues? Do you have an idea about what is causing the glitches? Could be a fault GPU?

My specs:

MacBook Pro 15"2018
Core i7 2.6ghz
16gb Ram
Radeon Pro 560X
macOS 10.14.1

External LG 27UD59-B (4k display) - connected through DisplayPort using Satechi USB-C to DisplayPort adapter


***Update: I changed the USBC to Display-Port Adapter to a simple USBC to DisplayPort cable and the issue is still there. So, it is not the cables.

And it ONLY happens in Safari. How weird

 

[precision01]

I'm not experiencing any kernel panic issues, neither audio issues. I still did not contact apple support, hoping it is a software issue.

 

[precision01]

Another update: Oddly, the issue only happens when External Display is set as "mirror displays", and scaled.
Also, the issue does not happen using Scaled>default. It only happens when using Scaled>Larger text with appearance of 2560 x 1440 (in the actual 4k display).