When I use a tool to make a change in my image, the changes don't show until I click on another tool. When I select something, the marching ants don't show up, even though it's selected (moving the cursor into the area makes the cursor change as it should). This just started happening yesterday. I disabled the Graphic Processor. On another Photoshop forum, others have the same issue. See this thread: Photoshop CC 2018: Image changes don't show on screen or in navigator | Photoshop Family Customer Community.
I am using the latest version of Photoshop (19.1). I am on El Capitan 10.11.6. I have an iMac 27" (Mid 2010).