Created attachment 1491175 [details]
This is a screenshot from Gnome Shell with settings app open and clicked on the clock to show the calendar.
Description of problem:
Zoom accessibility feature displays broken visual elements in the top-bar. Top-bar elements are rendered in half (only the top-half of the bar is visible) if the cursor is on top of the screen. Also tooltips and windows are not showing entirely, only portion of them.
This problem also affects GDM zoom feature.
Version-Release number of selected component (if applicable):
Always reproducible with gnome shell, gdm and Gnome Settings app.
This problem does not affect other applications like firefox, rhythmbox and nautilus: these applications are rendered correctly.
Steps to Reproduce:
1. Activate zoom feature, wither from accessibility icon or via settings app
2. Move the cursor around and notice the broken elements
Windows, tooltips and other visual elements are not displayed correctly. It seems their rendering are limited on a rectangle near the cursor.
Windows, tooltips and other visual elements should be displayed correctly.
I tested with Fedora 28 Workstation, same setup, and the problem does not verify.