Ever learn something and need to be able to find it later? Post it here for quick searchable access in the future.
As of VSCode 1.69, it has an excellent three-pane merge tool on par with the GitKraken and Beyond Compare merge tools. Here's how to configure Hg to use it (In Linux anyway, for other OS's you will have to keep searching...)
A little embarrassed that I did not know this, even though I have used PDFs for decades, and have always been frustrated with them because every time you re-open a PDF, you end up right at the first page of the document and have to try and remember where you were the last time. Really a pain when reading a long document or book that will take more than one sitting to get all the way thru.