GSHorizontalTypesetter improvements: Implement Truncation Modes Properly

[IOOI-SqAR]

I asked the Kimi AI how to further improve GSHorizontalTypesetter. Here is what it came up with:

https://www.kimi.com/share/19d34cdb-8242-8fc9-8000-00001fd46e3b

Implement Truncation Modes Properly

The current code treats all truncation modes as clipping. Implement proper head/middle/tail truncation:

case NSLineBreakByTruncatingTail: {
    // Calculate if truncation is needed
    if (position.x + glyphEntry->size.width > lineFragment->rect.size.width) {
        // Find truncation glyph (ellipsis) and fit it
        NSGlyph ellipsisGlyph = [font glyphWithName: @"ellipsis"];
        CGFloat ellipsisWidth = [font advancementForGlyph: ellipsisGlyph].width;
        
        // Remove glyphs until ellipsis fits
        while (index > firstGlyphIndex && 
               glyphEntry[-1].position.x + ellipsisWidth > lineFragment->rect.size.width) {
            index--;
            glyphEntry--;
        }
        
        // Insert ellipsis
        glyphEntry->glyph = ellipsisGlyph;
        glyphEntry->position = position;
        glyphEntry->size.width = ellipsisWidth;
        glyphEntry->dontShow = NO;
        
        lineFragment->lastGlyphIndex = index + 1;
        *newParagraph = NO;
        return NO;  // Line is complete
    }
    break;
}

[DTW-Thalion]

I looked into this. Confirming the current behaviour by laying out a 20-character run with no spaces in a 50-point container through an NSLayoutManager:

• word wrapping: 3 line fragments, all 20 glyphs shown (character wrapped)
• truncating tail: 1 line fragment, first 8 glyphs shown, the rest marked not-shown
• truncating head: 1 line fragment, also the first 8 glyphs shown

So all three truncation modes currently fall through to clipping (Source/GSHorizontalTypesetter.m around line 945): the glyphs that do not fit are hidden and no ellipsis is inserted. Head and middle also keep the wrong portion, since they keep the head like tail does.

The blocker for doing this properly is the ellipsis itself. Proper truncation inserts U+2026 at the head, middle or tail, but that glyph is not part of the text. This typesetter only positions glyphs that the layout manager has already generated for the characters; it does not synthesise glyphs. GSLayoutManager has no truncation-glyph support, so there is nowhere for the ellipsis to come from or be drawn. -replaceGlyphAtIndex: exists but overwriting a real glyph with an ellipsis would break the glyph-to-character mapping used by selection and hit testing.

So proper truncation is a cross-component change: the layout manager needs a way to represent and draw a truncation glyph, as AppKit does with its truncated-line handling, and the typesetter then decides where to cut. That is a larger design task than the snippet in the description suggests, and it cannot be done cleanly in the typesetter alone.

A smaller interim step would be to keep the correct portion per mode without an ellipsis (tail keeps the head, head keeps the tail, middle keeps both ends), but without the ellipsis indicator that may be more confusing than the current uniform clipping, so I would want your view first. @fredkiefer how would you like to approach this: add truncation-glyph support to the layout manager, or leave it as clipping for now?