Line end screen draw corruptions

Does anyone have this problem, or a solution for it? It’s happening on a newish iMac with latest versions of Sierra and Scrivener. I had this problem before on other machines and older versions of Scrivener, so I’m not sure where the problem lies. The document is short and resizing the window by pulling the corner reformats the text and is a quick fix, but after a couple of minutes of editing, these screen draw corruptions come back in. The text rewrapping and redrawing seems very slow. If I select a block of text, the right hand side is left out of the highlit area.

Could it be something to do with the font? It’s a very serviceable monospace font called Cousine, used in IA Writer. No problems with it in any other application. And the document gives no problems in Scrivener on the iPad - also using the Cousine font.

Previously tech support suggested a ‘corruption of the UI settings’ to be remedied by a reset: 'click the File menu and then hold down the Option key on your keyboard. Choose “Close Project and Clear Interface Settings”. Well, this does work, but I’m not sure it’s any different from simply closing and reopening the document.

Any help gratefully received. This is ruining my NaNoWriMo.

If you change the font, does the issue disappear?

If you copy the text to TextEdit and use the same font, do you have the same issue?

FWIW, I use Courier Prime and Menlo for monospaced writing.