It's GTFO. It's using an old version of the library from 2 years ago. https://www.curseforge.com/wow/addons/gtfo/issues/65
I found several addons explicitly setting L_UIDROPDOWNMENU_SHOW_TIME to 2. Increasing it to 20 in all cases made the menu usable. Files I changed: GTFO/Libs/LibUIDropDownMenu/LibUIDropDownMenu.lua HandyNotes_BrokenShore/Libs/LibUIDropDownMenu/LibUIDropDownMenu.lua Titan/libs/!LibUIDropDownMenu/LibUIDropDownMenu/LibUIDropDownMenu.lua TitanCurrenciesMulti/Core/ElioteDropDownMenu-1.0.lua
This fixes PX_put_data_long on 64-bit systems, where long int is 64 bits.
Attached is unified diff of edits needed to get rid of most of the warnings when built on CentOS 7.5. There's 3 remaining warnings about aliased pointers that need a little work but the code should work.
Build for 64-bit architectures