You can use CSS in your custom form to affect this behavior by lengthening the width of the field. Have you tried that yet? It may prevent this word wrap issue you're experiencing.
I looked into manipulating the CSS and custom forms. There are two problems this approach:
1. The picker is dynamically generated when the user clicks the combobox. When I went into the inspector view and extended the width of the parent combobox, the picker still had the subnets scrunched together. So even if I were to write a CSS to change the width from 500px to larger value, something is still making it look like what I posted earlier. I'm still under the impression the change needs to be made in the ExtJS code itself.
2. The subnet field for AWS blueprints is an integer pertaining to an identity in an IaaS database table. There is some custom logic being done in the out of the box form that is querying the DB and spitting out subnet names in the picker. I would need to re-invent the wheel to pull the subnets down from the DB through a custom action that queries the DB. There are several other fields that are also based on identities in the DB that would need to be accounted for as well.
I don't remember having this problem in 7.4. The GUI change seems to be locking the widths of these values somewhere. Since this is an out of the box IaaS blueprint type, I hope it is something that the VMware dev team can address.
You may really want to open an SR on this one so they can track it. Lastly, have you applied the latest hotfix for vRA 7.5? Not sure it'll address this issue, but it would address others.
We have 7.5 and use Mozilla (not the latest) and adding HF1 cleared up display issues for various menus. HF1 lists fixing I.E issues but also seems to fix Firefox display issues.
HF1 went on easy for me. Like the patch info in the 7.5 5480 management interface.
I deployed Hotfix 2 this afternoon on it and this GUI quirk still exists. Guess I'll have to wait for VMware to fix it...