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.

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.

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.

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.