Let's isolate the weird behavior you encounter when creating an invoice on your QuickBooks Online application.
The unresponsive page happens when an application encounter system issues due to a stored cache. Aside from restarting the application, you can also reset its data to refresh the connection. The steps to do this depend on your device:
Android: Go to More Options ⋮, tap Settings and select Refresh Data. Then, choose Yes.
IOS: From the Menu ☰, tap Help & Feedback and select Refresh Data. Then tap Refresh.
If you have the QuickBooks Online Desktop app, you can reset it by clicking the Help menu and then selecting Reset App Data.
If the issue persists, let's clear the stored junk files. Here's how:
Tap the File menu.
Choose Clear Cache.
Close the app.
Reboot your computer.
Reopen the application.
Once everything's good, you can check this reference on how you can customise the look of your invoice forms: