MarkdownDocument: one caret for every mode, pressed blocks kept across write-backs, preview read only, keyboard reach, history capped on every path

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

AGENTS.md

@@ -13,7 +13,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - Web views: `wry` 0.57 (Apache-2.0 OR MIT, default features off, macOS only) lays a child WKWebView over the window.
 - Terminal: `alacritty_terminal` 0.26 (Apache-2.0, default features off, native only) for the grid, its parser and the pseudo-terminal; `futures` carries its events. `vte` 0.15 (Apache-2.0 OR MIT), the parser it re-exports, reads `AnsiText`'s codes alone, so colored output builds for wasm32.
 - Diffs: `similar` 3.2 (Apache-2.0) for line and word diffs and three-way merges; its `unicode` feature splits words at punctuation.
-- Markdown: `markdown` 1.0.0 (markdown-rs, MIT) reads GitHub's flavor and math into a syntax tree with byte positions for `MarkdownRenderer`, `MarkdownEditor`, the highlights, block ranges and outline. `MarkdownRenderer` draws display math through `typography::Latex`, a ```` ```mermaid ```` block through `documents::Diagram`, and pictures from the owner's folder (`base`) or `data:` addresses (`base64` 0.22, MIT OR Apache-2.0); web pictures stay unloaded. Inline math shows as TeX: gpui text carries no inline pictures. `MarkdownDocument` holds a whole file in one `CodeEditor` (text and undo, colored by `CodeEditor::highlighter` with `markdown_highlights`): visual draws the file's top-level blocks in a gpui `list`, a pressed block opening as its source in a `TextInput` whose typing is the document's text at once and whose write-back is one undo step; source is the editor; preview draws without presses. Texts up to 128 KiB are read at once, longer ones on the background executor, and a new read keeps the list's blocks alike at both ends. `CodeEditor` keeps at most 64 MiB of undo text and drops the oldest steps past it. `BlockData::from_markdown` and its export still read with `pulldown-cmark` 0.13 (MIT, default features off); it adds GitHub's alerts, a callout's form, and reads each field's inline Markdown from the source by offset, as written.
+- Markdown: `markdown` 1.0.0 (markdown-rs, MIT) reads GitHub's flavor and math into a syntax tree with byte positions for `MarkdownRenderer`, `MarkdownEditor`, the highlights, block ranges and outline. `MarkdownRenderer` draws display math through `typography::Latex`, a ```` ```mermaid ```` block through `documents::Diagram`, and pictures from the owner's folder (`base`) or `data:` addresses (`base64` 0.22, MIT OR Apache-2.0); web pictures stay unloaded. Inline math shows as TeX: gpui text carries no inline pictures. `MarkdownDocument` holds a whole file in one `CodeEditor` (text and undo, colored by `CodeEditor::highlighter` with `markdown_highlights`): visual draws the file's top-level blocks in a gpui `list`, a pressed block opening as its source in a `TextInput` whose typing is the document's text at once and whose write-back is one undo step; source is the editor; preview draws without presses. Texts up to 128 KiB are read at once, longer ones on the background executor, and a new read keeps the list's blocks alike at both ends. One caret serves every mode: the editor's selection, which a written-back block moves; `set_caret` and `set_top_line` set caret and view apart, and a mode switch keeps the top line. A pressed block keeps its identity across the write-back of the one before it. Blocks are Tab stops that Enter or Space opens; preview takes no presses, undo or redo. `CodeEditor` keeps at most 64 MiB of undo and redo text, dropping the oldest undo and then the farthest redo after every record, composition, undo and redo. `BlockData::from_markdown` and its export still read with `pulldown-cmark` 0.13 (MIT, default features off); it adds GitHub's alerts, a callout's form, and reads each field's inline Markdown from the source by offset, as written.
 - Pictures: `image` 0.25 (MIT OR Apache-2.0), the crate gpui decodes with, default features off; it turns decoded frames and rims map tiles.
 - Math and diagrams: RaTeX 0.1.14 (`ratex-parser`, `ratex-layout`, `ratex-svg` with `embed-fonts`, `ratex-types`; MIT, KaTeX's fonts OFL) typesets `Latex` to SVG with outlined glyphs; `mermaid-rs-renderer` 0.3.1 (MIT, default features off) draws `Diagram`. Both go through gpui's `SvgRenderer` at the window's scale, loaded as gpui assets off the main thread; `usvg` 0.46, gpui's own, reads a diagram's size. A RaTeX or Mermaid panic on parsed input becomes the element's error. Each element holds its picture through `primitives::held`: a new source, or the element leaving the window, removes the old asset from gpui's cache and drops its image from the atlas after the frame.
 - Maps: Natural Earth's 110m countries (public domain). `assets/maps/countries.geojson`, trimmed by `scripts/countries.py` to an ISO code, a name and positions to 0.01°, backs `maps::WorldMap`; the gallery's tiles are cut from the same file, z0 to z3, by `scripts/tiles.py` with Pillow.