elements in Preview "float" away and are hard to control

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Hello dorian444,

Welcome to Apple Support Communities. We understand you have some issues with Preview while annotating PDF forms. What you describe with the text or lines "floating or sliding" away is not expected behavior. We'd like to have you walk through some steps to help narrow down the possible underlying cause so we can work to resolve this for you. We want you to create a test user profile on your Mac, log in to it, then test Preview with a PDF.

This will help rule out if there is something related to your user profile causing this issue in Preview. Reply back with how the test goes.

Set up users, guests, and groups on Mac - Apple Support

Also, for future reference, if you ever need to submit feedback to us, you can use our official form here: Product Feedback - Apple

For any technical support issues, feel free to reach out to our teams directly any time: Apple Support

We look forward to hearing how it goes.

Warm regards.