There is a little gripe I have with indent-heuristic = simple while using the "tabs to indent, spaces to align" styling. For C functions, when parameters are long, I tend to do it like this:

static int my_func(int parameter1,
                   bool parameter2,
                   struct sth *parameter3)
/* ^^^^^^^^^^^^^^^
 * These are all spaces, to align the parameters with
 * the characters of the function header. */
{
        /* And the content is then indented with tabs. */
        /* snip */
}

With indent = { tab-width = 8, unit = "\t" }, when I add a new line to the parameters, Helix matches the previous indent by putting as many tabs as can fit, then padding the rest with spaces, like so (with . being spaces, <------> being tabs):

static int my_func(int parameter1,
...................bool parameter2,
<------><------>...bool new_parameter, /* added here with o */
...................struct mystruct *parameter3)
{
        /* snip */
}

It would be nice to have an indent-heuristic that simply copies the same exact whitespace onto the new line instead - where there are all tabs, copy tabs; where there are all spaces, copy spaces; if there is some weird mixture, just copy the mixture - i.e. to be even less clever about it.

If there is interest, I am willing to author a PR for this, be it by adding a new special heuristic, or changing the existing simple one.