Glyph count used a grapheme cluster count in text input hit testing

[SimonSapin]

From #23272 (comment)

The script::textinput::TextInput::set_edit_point_index method is called for click events on <input> elements to set the cursor’s position. It takes a usize parameter that is interpreted as a number of grapheme clusters:

servo/components/script/textinput.rs

Lines 990 to 996 in 576394b

	pub fn set_edit_point_index(&mut self, index: usize) {
	let byte_size = self.lines[self.edit_point.line]
	.graphemes(true)
	.take(index)
	.fold(0, |acc, x| acc + x.len());
	self.edit_point.index = byte_size;
	}

This number ultimately comes from here:

servo/components/gfx/text/text_run.rs

Lines 369 to 385 in 576394b

	/// Returns the index in the range of the first glyph advancing over given advance
	pub fn range_index_of_advance(&self, range: &Range<ByteIndex>, advance: Au) -> usize {
	// TODO(Issue #199): alter advance direction for RTL
	// TODO(Issue #98): using inter-char and inter-word spacing settings when measuring text
	let mut remaining = advance;
	self.natural_word_slices_in_range(range)
	.map(|slice| {
	let (slice_index, slice_advance) = slice.glyphs.range_index_of_advance(
	&slice.range,
	remaining,
	self.extra_word_spacing,
	);
	remaining -= slice_advance;
	slice_index
	})
	.sum()
	}

servo/components/gfx/text/glyph.rs

Lines 576 to 600 in 576394b

	// Scan the glyphs for a given range until we reach a given advance. Returns the index
	// and advance of the glyph in the range at the given advance, if reached. Otherwise, returns the
	// the number of glyphs and the advance for the given range.
	#[inline]
	pub fn range_index_of_advance(
	&self,
	range: &Range<ByteIndex>,
	advance: Au,
	extra_word_spacing: Au,
	) -> (usize, Au) {
	let mut index = 0;
	let mut current_advance = Au(0);
	for glyph in self.iter_glyphs_for_byte_range(range) {
	if glyph.char_is_space() {
	current_advance += glyph.advance() + extra_word_spacing
	} else {
	current_advance += glyph.advance()
	}
	if current_advance > advance {
	break;
	}
	index += 1;
	}
	(index, current_advance)
	}

… where is is computed as a number of glyphs.

Grapheme clusters only map to glyphs one to one in simple cases. This could cause the cursor to end up at an incorrect position, far from where click cursor was.

Instead, the gfx code should map positions in the glyph sequence back to UTF-8 positions in the original Unicode text, and set_edit_point_index interpret its parameter as a UTF-8 byte count instead of a grapheme cluster count.