Reproduction steps:
1) Download and open "smallrepro.zip" project
2) Select sprite, located in Assets folder (there's a single sprite)
3) Press on "Sprite Editor" button in order to open sprite editor
4) Select any input field in the left bottom corner (for example select "Border; L" field
5) Press "tab" button
Notice that cursor stays in the same input field

Expected: It moves to another input field

Regression appeared in 2018.2.0a4
Reproduced with: 2018.2.0a4, 2018.2.0b1
Unable to reproduce with: 2018.2.0a3, 2018.1.0f1, 2017.4.0f1

Reproduced on MacOS and Windows

Note: Seems like only Sprite Editor is affected. I still can switch input fields in Inspector and Build Settings windows