The solid viewport my model only visible in an area. other viewport still fine.
Ah yes, a classic one. You accidentally at some point enabled clipping region. Just press ALT + B to clear it.
Indeed!
You can only see between the yellow lines and the Viewport is a bit lighter between those lines (but that might be hard to see in the screenshot):
Ah Thank you. So there are Clipping region and Render Region. I tried Ctrl Alt B like the last video but it didn't work.
Clipping Region can be very handy, when modeling something like the interior of a car, for instance.
And yes, ALT+B is used to both Set and Clear the Clipping Region.