Thank you, your first "guess" solves the problem - excellent! For anyone using the Noodlesoft version, the actual code I've added is related to the creation of the NoodleLineNumberView using initWithScrollView:(NSScrollView *)aScrollView. After sending that, I just tweak the custom view as suggested; this is also backwards-compatible which is helpful (at least to me). Note that the line numbers actually live in a vertical ruler. lineNumberView = [[NoodleLineNumberView alloc] initWithScrollView:scrollView]; [scrollView setVerticalRulerView:lineNumberView]; [scrollView setHasVerticalRuler:YES]; [[scrollView verticalRulerView] setClipsToBounds:YES]; // This defaults to NO as of macOS 14 (Sonoma), we need YES to contain its drawing // etc This seems to be a complete fix. I don't remember seeing the AppKit Release Notes, they're very useful although I may not have made the connection without some help. Hopefully there will be some along soon for macOS 15. Thanks again to Ziqiao Chen for the help.

In reply to The Eskimo, 6 days ago: Sorry for the delay in replying, I've been away. OK, I thought I had pressed the disclosure triangle to show the 8 identical errors but I've found it now (below). Note that I have changed my CIImage code and am now using CGImage but with the same issue. Xcode is 14.2, I have no earlier reference as it's new code. As you can probably see, the code is in a method run from a block invoked by a Save panel when the user clicked on 'Save'. Since the backtrace does include my code, Perhaps I can't just ignore it; it seems to be CGImageDestinationFinalize causing the issue. Odd that I get 8 copies of the report though.

Here's another datum. I propose to follow the "it’s likely that this isn’t your problem and you should feel free to ignore the issue" advice but this is an occurrence not involving WKWebView. I get no less that 8 occurrences of the warning from a use of UIContext. Though I may agree it would be better not to use it on the main thread, 8 warnings seems a little excessive. I did try obtaining the NSURL separately into a variable fed into this statement but (unsurprisingly) that made no difference. Xcode 14.2, macOS Ventura 13.2, targeting macOS 10.14.

grb2007's answer is immensely helpful. However, I still have some confusion here: If I OPTION+CLICK on an item in the File Navigator, that does not open up a new document tab as Developer Tools Engineer  suggests but rather opens a new editor at the bottom of the same temporary (italicised) tab. There are several references here to the Xcode documentation for this feature. Can anyone provide a link please because I can't find it.