Cannot align with non-monospaced font?

Hello, 

I am on a custom language plugin development. When I format my code with Alignment class, everything is fine if I use monospaced font. 

However, If I choose Times New Roman font, the code cannot aligned. It would look like this:

tmp2    <= 1'b0;
tmp    <= 1'b0;

The first line has an extra space before '<='.

Is there a way to make the code aligned under all kinds of fonts? what should I do? Or is this a bug?