Synchronised Scrolling: a brief movie on PDF

A few days ago, I reported a bug in the handling of PDF documents in macOS Mojave 10.14.3, which linked open windows together so that they bizarrely scrolled in unison.

Here is a demonstration in a screenshot movie:

You are watching six quite different PDF documents, each open in its own window in my free PDF reader Podofyllin, which uses vanilla macOS features (PDFKit) to handle and display those documents. I have provided source code to a very similar app which you can study and build for yourself here: BasicPDFViewer 1.0b3.

In this demonstration, the centre window in the lower row is at the front. I then use the cursor keys to step through its pages. As the selected thumbnail view changes in that window, the bug changes the selected page in all the other five windows. I step down through the document, and back up again, with every window changing in unison.

This is entertaining viewing, so long as you aren’t sensitive to screen motion, but makes the thumbnail views in macOS useless. Every app which displays PDF thumbnails – including Apple’s own Preview – therefore has to use its own workaround, because Apple doesn’t apparently consider that fixing this bug in PDFKit is important enough.

Please share this widely if you feel that, in return for Apple’s premium prices, we should get premium products, without such gross bugs.