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shaping: Don't assume there's a space glyph when rendering tabs (#32979)

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Previously if a font didn't have a space advance and it was needed to
make advances for tabs, Servo would try to read the advance from the
font. If the font didn't have a space glyph, Servo would panic. This
fixes that issue by making the space advance part of the `FontMetrics`
of a font (like Gecko) and falling back properly if that glyph doesn't
exist. The rendered glyph is still the "space" glyph, but we make
sure to select a font that supports that glyph explicitly.

This prevents a crash, but tabs still aren't handled properly. In
reality, tab stops should be calculated in layout and the size of
the space character of the current font shouldn't come into play.
The addition of the space advance metric will make this easier.

Fixes #32970.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
This commit is contained in:
Martin Robinson 2024-08-23 04:17:44 -07:00 • committed by GitHub
parent ebdae6094e
commit 78e2691d3f
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7 changed files with 75 additions and 40 deletions
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37
components/fonts/font.rs
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@ -12,6 +12,7 @@ use std::{iter, str};
use app_units::Au;
use bitflags::bitflags;
use euclid::default::{Point2D, Rect, Size2D};
use euclid::num::Zero;
use log::debug;
use malloc_size_of_derive::MallocSizeOf;
use parking_lot::RwLock;
@ -137,6 +138,9 @@ pub struct FontMetrics {
pub line_gap: Au,
pub zero_horizontal_advance: Option<Au>,
pub ic_horizontal_advance: Option<Au>,
/// The advance of the space character (' ') in this font or if there is no space,
/// the average char advance.
pub space_advance: Au,
}
impl FontMetrics {
@ -144,20 +148,21 @@ impl FontMetrics {
/// no font can be found.
pub fn empty() -> Self {
Self {
underline_size: Au(0),
underline_offset: Au(0),
strikeout_size: Au(0),
strikeout_offset: Au(0),
leading: Au(0),
x_height: Au(0),
em_size: Au(0),
ascent: Au(0),
descent: Au(0),
max_advance: Au(0),
average_advance: Au(0),
line_gap: Au(0),
underline_size: Au::zero(),
underline_offset: Au::zero(),
strikeout_size: Au::zero(),
strikeout_offset: Au::zero(),
leading: Au::zero(),
x_height: Au::zero(),
em_size: Au::zero(),
ascent: Au::zero(),
descent: Au::zero(),
max_advance: Au::zero(),
average_advance: Au::zero(),
line_gap: Au::zero(),
zero_horizontal_advance: None,
ic_horizontal_advance: None,
space_advance: Au::zero(),
}
}
}
@ -517,6 +522,14 @@ impl FontGroup {
codepoint: char,
next_codepoint: Option<char>,
) -> Option<FontRef> {
// Tab characters are converted into spaces when rendering.
// TODO: We should not render a tab character. Instead they should be converted into tab stops
// based upon the width of a space character in inline formatting contexts.
let codepoint = match codepoint {
'\t' => ' ',
_ => codepoint,
};
let options = FallbackFontSelectionOptions::new(codepoint, next_codepoint);
let should_look_for_small_caps = self.descriptor.variant == font_variant_caps::T::SmallCaps &&