It appears to me from the property name and the VS Code implementation that triggerCharacters only supports single characters in the array. Support for whole sequences would be very helpful for languages like PHP and C++, that have multi-character operators:

TestClass::staticMethod(); $obj->property;

I would like to trigger completion on -> and ::, but of course not on a single > (greater-than operator) or : (no meaning).
Currently, sending ["::", "->"] to VS Code seems to result in VS Code only triggering on the first character, : or -. I therefor send : and > instead.
Please document that the array can not only contain characters, but string sequences.