Affected Version: 2.15.1-4054-gc98e654fbb

What steps will reproduce the problem?
1. Load https://chromium-review.googlesource.com/c/chromium/src/+/508293
2. Click "Choose trybots"
3. Type "linux" in the textfield
4. Click the checkbox for the first bot
5. Type "mac" in the textfield

What is the expected output?
No bots checked in the mac list

What do you see instead?
First mac bot is already checked

Please provide any additional information below.