Re: [PATCH 03/11] vt: properly support zero-width Unicode code points

Nicolas Pitre
2025-04-15T19:06:03Z

On Mon, 14 Apr 2025, Jiri Slaby wrote:

> On 10. 04. 25, 3:13, Nicolas Pitre wrote:
> > From: Nicolas Pitre <npitre@baylibre.com>
> >
> > Zero-width Unicode code points are causing misalignment in vertically
> > aligned content, disrupting the visual layout. Let's handle zero-width
> > code points more intelligently.
> ...
> > --- a/drivers/tty/vt/vt.c
> > +++ b/drivers/tty/vt/vt.c
> > @@ -443,6 +443,15 @@ static void vc_uniscr_scroll(struct vc_data *vc,
> > unsigned int top,
> > }
> > }
> >
> > +static u32 vc_uniscr_getc(struct vc_data *vc, int relative_pos)
> > +{
> > + int pos = vc->state.x + vc->vc_need_wrap + relative_pos;
> > +
> > + if (vc->vc_uni_lines && pos >= 0 && pos < vc->vc_cols)
>
> So that is:
> in_range(pos, 0, vc->vc_cols)
> right?

Good idea. Didn't know about that one.

> > if (vc->vc_utf && !vc->vc_disp_ctrl) {
> > - if (ucs_is_double_width(c))
> > + if (ucs_is_double_width(c)) {
> > width = 2;
> > + } else if (ucs_is_zero_width(c)) {
> > + prev_c = vc_uniscr_getc(vc, -1);
> > + if (prev_c == ' ' &&
> > + ucs_is_double_width(vc_uniscr_getc(vc, -2))) {
> > + /*
> > + * Let's merge this zero-width code point with
> > + * the preceding double-width code point by
> > + * replacing the existing whitespace padding.
> > + */
> > + vc_con_rewind(vc);
> > + } else if (c == 0xfe0f && prev_c != 0) {
> > + /*
> > + * VS16 (U+FE0F) is special. Let it have a
> > + * width of 1 when preceded by a single-width
> > + * code point effectively making the later
> > + * double-width.
> > + */
> > + } else {
> > + /* Otherwise zero-width code points are
> > ignored */
> > + goto out;
> > + }
> > + }
>
> Please, extract this width evaluation to a separate function.

Done.