Just a note to my future self... I've been doing some Arduino programming with the ESP8266+Oled board I purchased recently and went back to the Visual Studio Code PlatformIO environment after a break and ran into some weird behaviour including this error:
"Conflicted extensions with IntelliSense service were detected (Arduino). Code-completion, linting and navigation will not work properly."
Yes, I had all sorts of problems and could kind of get it to work by quitting and re-starting VSC.
When getting set up the first thing I'd done was install the Arduino extension and it turns out that this is not only not needed but causes problems. Remove the Arduino extension so you have C/C++ and PlatformIO IDE.
After importing my Arduino .ino file I converted it to "proper" C++ and all works well.