Double-click selection is wonky

[snej]

Selection behavior of double-clicking in Go code is weird and annoying in some contexts:

• Anywhere in a function parameter list, it selects the entire parameter list (to just inside the parens.)
• In an initializing assignment, like foo := bar(), double-clicking foo also selects the space after it.
• In a top-level variable declaration, like var foo = 0, double-clicking foo also selects the space before it.

This must be due to some subtlety of the language-grammar file, but I don't know nearly enough about those to be able to diagnose the problem.