The Bug:

The screen flashes when attempting to open GUIs while standing inside nether portals.

This issue is not a duplicate of MC-175373. MC-175373 talks about how you're able to open GUIs while standing inside nether portals; this ticket (MC-261862) focuses on the aspect of the screen flashing.

This is a problem because the rapid flashing may disorient those who suffer from photosensitive epilepsy.

Steps to Reproduce:

1. Construct a nether portal and place a chest beside it near the portal blocks.
2. Switch to survival mode, stand in the nether portal, and attempt to open the chest constantly.
3. Watch your screen closely as you do this.
4. Take note as to whether or not the screen flashes when attempting to open GUIs while standing inside nether portals.

Observed Behavior:

The screen flashes.

Expected Behavior:

The screen would not flash, thus significantly reducing the chances of those who suffer from photosensitive epilepsy from experiencing seizures as a result of the flashing.