When I use display zoom, there are diagonal line on some app

Hi Nsidik,

Thanks for contacting Apple Support Communities!

We understand that you see a thin line when using your drawing apps. Are you using an Apple Pencil in the apps where you experience this behavior? We ask because this can happen if the Apple Pencil tip needs to be replaced or tightened. You can learn more here: Use Apple Pencil with your iPad - Apple Support

"Learn more

• If your Apple Pencil isn't drawing as expected, make sure that its tip isn't loose. If it is, twist it to the right to tighten it.
• Use Markup in apps like Mail, Messages, and Photos to add drawings, text, and shapes with your iPad and Apple Pencil. For example, you can add a signature to a PDF.
• Learn how to create an instant note from the Lock screen or pick up where you left off on your last note."

If not, you can also update the apps or delete and reinstall the apps: If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support

We look forward to your reply.

Cheers!

Nsidik,

Thanks for following up. In this case we'd recommend reaching out for additional support here: Get Support

Take care.