The bug

The Social Interactions UI is not horizontally centered. In full screen mode on a screen at 1440x900 resolution with GUI scale set to 3 (reproducible at most resolutions and all GUI scales), there are 372 pixels to the left side, but only 360 to the right.

How to reproduce

1. Set your GUI scale to 3 and your resolution to 1440x900 (not necessary to reproduce the bug, but necessary to obtain the exact same measurements)
2. Open a world to LAN or join a server in 1.16.4 prerelease 2 or above
3. Open the Social Interactions menu
4. Take a screenshot by pressing F2
5. Go to your screenshots folder
6. Open the screenshot in a photo editor that shows dimensions when selecting an area (Preview on macOS)
7. Select the area from the left edge of the screenshot to the left edge of the GUI and notice how there are 372 pixels
8. Select the area from the right edge of the screenshot to the right edge of the GUI and notice how there are 360 pixels
9. Compare the two values, and notice how there is more space on the left than on the right.

Media

scale2.png: Taken using GUI scale 2.

Notes

This issue is extremely difficult to notice without looking for and measuring the dimensions.

If this is a duplicate - I am very sorry. I searched for a while though.