When you're in the creative inventory, are looking in a random tab and want to go to the search tab, you usually press T to open the search tab. In this snapshot, the letter "T" is also put.

How to reproduce:

1. Open a world in creative mode
2. Open the inventory and go to a tab which is not the Search tab
3. Press T

What should happen:
You should enter the search tab with an empty search box.

What happens:
You enter the search tab, with the letter T already filled.