A few days ago, I reported an error handling PDF documents in macOS Mojave 10.14.3, which linked open windows together so that the bizarre rolled together.
Here's a demonstration in a screen capture image:
You see six quite different PDF documents, each open in a separate window in my free PDF reader Podofyllin, which uses vanilla macOS features (PDFKit) to handle and view these documents. I've given the source code to a very similar app that you can study and build for yourself: BasicPDFViewer 1
In this demonstration, the middle window in the bottom row is front. I then use the arrow keys to go through the pages. When the selected thumbnail view changes in that window, the error changes the selected page in all the other five windows. I go down the document and back up again, with all the windows changing.
This is entertaining display as long as you are not sensitive to the screen movement, but makes the thumbnails in macOS useless. Each app that displays PDF thumbnails – including Apple's own preview – must therefore use its own solution, as Apple apparently does not consider it important to fix this error in PDFKit.
Please share this much if you feel it, in