How does one Print all WKWebView On AND Offscreen content OSX and iOS

I’ve successfully used the SPI -[WKWebView _printOperationWithPrintInfo:] passing the usual [NSPrintInfo sharedPrintInfo].
Note that you CAN’T use -runOperation on the returned NSPrintOperation. You must use -runOperationModalForWindow:…. which is quite similar. The problem resides in the WebKit internals that expects a running runloop and a preview to be made internally to know the number of pages.

It definitely works with offscreen content, if what you mean by offscreen is “not fully displayed on screen”. I still have a WKWebView displayed in a window, but it’s very tiny and only displays a very short fraction of the entire webview content (21 A4 pages!). Hope this helps!

PS: Tested on 10.12, 10.14 and 10.15.
Code is like this:

     SEL printSelector = NSSelectorFromString(@"_printOperationWithPrintInfo:"); // This is SPI on WKWebView. Apparently existing since 10.11 ?
     
     NSMutableDictionary *printInfoDict = [[[NSPrintInfo sharedPrintInfo] dictionary] mutableCopy];
     printInfoDict[NSPrintJobDisposition] = NSPrintSaveJob; // means you want a PDF file, not printing to a real printer.
     printInfoDict[NSPrintJobSavingURL] = [NSURL fileURLWithPath:[@"~/Desktop/wkwebview_print_test.pdf" stringByExpandingTildeInPath]]; // path of the generated pdf file
     printInfoDict[NSPrintDetailedErrorReporting] = @YES; // not necessary         

     // customize the layout of the "printing"
     NSPrintInfo *customPrintInfo = [[NSPrintInfo alloc] initWithDictionary:printInfoDict]; 
     [customPrintInfo setHorizontalPagination: NSPrintingPaginationModeAutomatic];
     [customPrintInfo setVerticalPagination: NSPrintingPaginationModeAutomatic];
     [customPrintInfo setVerticallyCentered:NO];
     [customPrintInfo setHorizontallyCentered:NO];
     customPrintInfo.leftMargin = 0;
     customPrintInfo.rightMargin = 0;
     customPrintInfo.topMargin = 5;
     customPrintInfo.bottomMargin = 5;
  
     NSPrintOperation *printOperation = (NSPrintOperation*) [_webView performSelector:printSelector withObject:customPrintInfo];

     [printOperation setShowsPrintPanel:NO];
     [printOperation setShowsProgressPanel:NO];

//    BOOL printSuccess = [printOperation runOperation]; // THIS DOES NOT WORK WITH WKWEBVIEW! Use runOperationModalForWindow: instead (asynchronous)
     [printOperation runOperationModalForWindow:self.window delegate:self didRunSelector:@selector(printOperationDidRun:success:contextInfo:) contextInfo:nil]; // THIS WILL WORK, but is async