Hi Steve,
just released a new version of my GuiTools Framework.
Before I release things, I usually check if they properly compile in the historical SDL 0.954 version and the latest stable new GL versions of QB64 (both, 32/64 bits).
This time I added your SDL 2020 updated version from your repo in my testing chain for the 1st time and found that the IDE will crash after a few seconds when syntax checking any loaded GuiTools application (from the src_XXX folders). As far as I could find out, it must be some recursion bug in the $INCLUDE handling, because if I paste the include's contents into the sources (in place of the $INCLUDE lines) to put everything together in one big file, then it works fine and the programs will also properly compile.
Maybe you can have a look at this, if your time allows, no need to hurry...