For whatever reason, the create new tab button doesn't extend all the way to the top of the window.
Using Opera GX in full screen, there are at least 3 pixels above the "new tab" button. resulting in it being very easy to click above it and wonder why you didn't get a new tab.
Chrome, Firefox, & Edge all extend to the top of the window.
Considering Opera is a Chromium-based browser, it was must have been edited to be this annoying.
Every single other button and UI element extends to the top of the window.
Every. Single. One.
Except for creating new tabs