Function shape_visual_items 

pub fn shape_visual_items<T: ParsedFontTrait>(
    visual_items: &[VisualItem],
    font_chain_cache: &HashMap<FontChainKey, FontFallbackChain>,
    fc_cache: &FcFontCache,
    loaded_fonts: &LoadedFonts<T>,
    debug_messages: &mut Option<Vec<LayoutDebugMessage>>,
) -> Result<Vec<ShapedItem>, LayoutError>

Shape visual items into ShapedItems using pre-loaded fonts.

This function does NOT load any fonts - all fonts must be pre-loaded and passed in. If a required font is not in loaded_fonts, the text will be skipped with a warning.

Optimization: Inline Run Coalescing

When consecutive text VisualItems share the same layout-affecting properties (font, size, spacing, etc.) but differ only in rendering properties (color, background), they are coalesced into a single shaping call. This dramatically reduces the number of font.shape_text() invocations for syntax-highlighted code where hundreds of <span> elements use the same monospace font but different colors. After shaping, the original per-span styles are restored to each ShapedCluster based on byte-range mapping.