To replicate, on the mobile device start a new Topic
The create topic section will now have 3 inputs
Tap on the category, a dropdown list should open, but instead the keyboard will pop and the drop down will disappear immediately.
You can tap it repeatedly but it will keep on closing.
A partial possible fix:
The dropdown should not have the input box focus by default which will allow the user to first scroll through the options.
that will not prompt the keyboard to pop. if the user would like to search for topic then he can set focus on the input manually.