Reproduction steps:
1. If needed return/revoke Unity Licence (you need to see Activation screen)
2. Restart Unity
3. In activation screen try to enter serial number by hand

Expected result: user is able to enter serial number by hand
Actual result: serial number string is selected on every modification

Notice that after writing a letter Unity immediately checks if licence is legitimate and whole serial number string gets selected. When user will write another character whole string will be overwritten.

Repro on 5.3.5f1, 5.3.6f1
No repro on 5.3.0f4, 5.3.4f1, 5.4.0f2