Given that Geany does not know what parser is going to be used my feeling is that Geany should not _by default_ encourage something that might not be legal.
I'm rather thinking the contrary, that Geany should support wider use cases so long as it doesn break another one. And thus I would think the parser should accept leading whitespaces as well.
And I don't think that we can rely on this file type to guarantee any particular syntax, for example the parser accepts `;` comments, and I didn't look again but I guess the lexer has similar extension that breaks your expectations of the highlighting being a validator or similar.