The load-bearing design contract of the Edmund Markdown editor. Load BEFORE any non-trivial code change in this repo; when asking "why is it built this way"; before proposing a new mechanism, subsystem, or refactor; whenever you are tempted to insert/strip display characters, use NSTextAttachment, touch NSTextView.layoutManager, store NSTextBlock/NSTextTable attributes, or add a new overlay/decoration; and before designing anything that syncs storage, selection, undo, or the viewport. Covers the two hard invariants (storage == rawSource; TextKit 2 only), the render pipeline, edit/undo flow, the TextKit 2 drawing model, the read-mode contract, and the known weak points. Not for build/run/release mechanics, debugging triage, or live-repro drivers — see "When NOT to use this skill".
Installation
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The load-bearing design contract of the Edmund Markdown editor. Load BEFORE any non-trivial code change in this repo; when asking "why is it built this way"; before proposing a new mechanism, subsystem, or refactor; whenever you are tempted to insert/strip display characters, use NSTextAttachment, touch NSTextView.layoutManager, store NSTextBlock/NSTextTable attributes, or add a new overlay/decoration; and before designing anything that syncs storage, selection, undo, or the viewport. Covers the two hard invariants (storage == rawSource; TextKit 2 only), the render pipeline, edit/undo flow, the TextKit 2 drawing model, the read-mode contract, and the known weak points. Not for build/run/release mechanics, debugging triage, or live-repro drivers — see "When NOT to use this skill".
Edmund architecture contract
Edmund is a native macOS Markdown editor with live preview: AppKit +
TextKit 2, SwiftPM, macOS 14+. Two targets (Package.swift):
Target
Role
EdmundCore
Library: parsing, rendering, EditorTextView, all tests. Most work happens here.
edmd
Executable: NSDocument app shell, Settings (SwiftUI), menus. Note: edmd is the Mach-O binary name; the app is "Edmund".
Project ambition (maintainer, 2026-07-05): product-first — "the CotEditor of
Markdown editors". Bias toward polish of the editing experience over feature
count. The hardest live problem class to date is delete-drift
(caret/selection integrity, 6 investigation rounds); the costliest failures
were delete-drift and undo/redo viewport drift. Every rule below traces to one
of those scars.
Ground truth this file distills: docs/ARCHITECTURE.md (repo root).
Treat that doc as authoritative if the two ever disagree, and fix this skill.
Glossary (each term defined once)
rawSource — the document's Markdown text, the single source of truth
(EditorTextView.rawSource).
storage — the NSTextStorage the text view displays
(EditorTextStorage, Sources/EdmundCore/TextView/EditorTextStorage.swift).
Block — one logical Markdown block (paragraph, heading, list run, code
fence, table, quote/callout run). Model: Sources/EdmundCore/Model/Block.swift.
Active block — the block under the caret; it renders its raw markdown
(delimiters visible/editable) while all others render styled.
Recompose — restyling storage from blocks
(Sources/EdmundCore/TextView/EditorTextView+Composition.swift).
Fragment — an NSTextLayoutFragment, TextKit 2's per-paragraph layout
unit. Off-screen fragments have estimated heights until laid out.
Overlay — an image or stroked path drawn at a character's laid-out
position by the custom fragment class (see §4), replacing what
NSTextAttachment would do in TextKit 1.
Delete-drift — the bug class where rawSource/storage/selection desync
and every later edit lands the caret in the wrong place. Chronicle:
docs/investigations/delete-drift-investigation.md.
IME composition — an input method's provisional "marked text"
(hasMarkedText()), present in storage before the user commits it.
1. The two non-negotiable invariants
Break either and the editor misbehaves in subtle, delayed ways. Every design
review starts here.
The displayed text storage is always character-identical to rawSource.
Rendering only ever adds/changes attributes. Delimiters (**, `,
[!note], …) are hidden, never stripped: hiddenFont (0.01pt system
font, Sources/EdmundCore/Rendering/EditorTextView+Rendering.swift:55) plus a
clear foregroundColor makes them invisible without touching the string.
Rationale. Identity mapping between display offsets and raw offsets means
there is no offset-translation layer — caret math, selection, undo diffs,
incremental reparse, and autosave all operate on one coordinate system.
Consequences you must respect:
Consequence
Why
No NSTextAttachment, ever
TextKit 2 only honors attachments on U+FFFC (OBJECT REPLACEMENT CHARACTER), which rawSource never contains. Images, math, bullets, checkboxes, icons are drawn as overlays instead (§4).
No inserted display characters
A synthesized <br>, bullet glyph, or padding character would desync offsets. Use attributes (.kern, paragraph styles) or overlays.
Never mutate storage while IME is composing
During composition, storage holds marked text so the invariant is transiently false and didChangeText defers syncing. Styling that runs beginEditing/setAttributes/invalidateLayout mid-composition strands the marked text; the invariant then stays broken and every later edit drifts the caret. Every storage-touching styling path — including async ones scheduled before composition began — must guard !hasMarkedText().
The incident. The original delete-drift bug: an async restyle fired during
IME composition, stranded the marked text, didChangeText kept bailing on its
own guard, and the invariant stayed silently broken — caret drift on every
subsequent edit. becomeFirstResponder now resyncs from storage as a
catch-all. Full write-up: docs/investigations/delete-drift-investigation.md.
Invariant 2 — TextKit 2 only
Never touch NSTextView.layoutManager (the TextKit 1 NSLayoutManager
accessor) and never store NSTextBlock/NSTextTable attributes. Either one
silently and permanently reverts the view to TextKit 1 — no error, no log,
just different (and wrong-for-us) layout from then on.
Rationale. All custom drawing rides NSTextLayoutFragment subclassing
(§4), and viewport-based layout (only on-screen content laid out) is what
makes large documents fast. Both are TextKit 2 facilities; a TK1 fallback
kills them.
The tripwire. DEBUG builds observe
NSTextView.willSwitchToNSLayoutManagerNotification and assertionFailure
if the fallback ever triggers —
textKit1FallbackTripwire(_:),
Sources/EdmundCore/TextView/EditorTextView.swift:272-303. If you see that
assertion, some code path you touched used a TK1 API or attribute. Find it;
do not suppress the assert.
Corollary: Edit-mode tables cannot use NSTextTable. Alignment is done by
distributing slack via .kern on hidden pipe glyphs —
Sources/EdmundCore/Rendering/EditorTextView+TableRendering.swift.
2. Render pipeline
rawSource ──BlockParser──▶ [Block] ──styleBlock per block──▶ attributed runs in storage
│
└─ SyntaxHighlighter (swift-markdown walker
+ custom parsers: callouts, ==highlight==,
wikilinks, comments, footnotes, math,
backslash escapes, inline HTML tags)
Recompose entry points (pick the narrowest that works — a full recompose
resets every fragment height to an estimate, see §6):
Function
Scope
Used for
recompose(cursorInRaw:)
Whole document
Load, indent — never for undo (§3)
recomposeDirty(_:cursorInRaw:)
A set of block indices, in place
The workhorse; attribute-only
recomposeIncremental(cursorInRaw:...)
The block(s) the caret moved between
Most cursor moves
recomposeReplacing(oldRange:with:...)
One contiguous text span
Undo/redo restore
Lazy styling (Sources/EdmundCore/TextView/EditorTextView+LazyStyling.swift):
a large dirty set styles only the viewport synchronously; the rest is finished
by the idle drain (time-budgeted main-thread slices) and scroll
promotion (style blocks as they enter the viewport).
Gotcha: attribute-only changes do not re-measure geometry in TextKit 2.
If a restyle changed a block's height/indent, call invalidateLayout(for:)
on its range or the fragment keeps a stale frame. recomposeDirty and the
idle drain already do this; any new path must too.
3. Edit flow & undo
Normal edit: shouldChangeText (records a coalesced undo snapshot) →
NSTextView mutates storage → didChangeText syncs rawSource and restyles
the edited block(s) (Sources/EdmundCore/TextView/EditorTextView+EditFlow.swift,
+Composition.swift). Edits capture a pendingEdit on EditorTextStorage
and reparse a window, not the whole document.
Undo/redo is custom (Sources/EdmundCore/TextView/EditorTextView+Undo.swift):
stacks of rawSource snapshots, bypassing NSTextView's built-in undo.
Restoring diffs the snapshot against current text (textDiff(old:new:),
single contiguous span) and applies it with the range-bounded
recomposeReplacing — never a full recompose, because a full recompose
resets every fragment to a TextKit 2 height estimate and the follow-up scroll
lands wrong (this was the undo/redo viewport-drift failure). The changed text
drives the viewport: hold if any of it is on-screen, else center it.
AppKit does NOT pair every storage mutation with didChangeText. Proven
incident (delete-drift round 4): a drag-move of selected text dropped on no
valid target deletes the dragged range via shouldChangeText →
replaceCharacters and never calls didChangeText — silently freezing
rawSource/blocks; every later edit drifts the caret and autosave writes
stale text. The heal: shouldChangeText schedules a next-run-loop
bypass check (scheduleBypassedEditSyncCheck, +EditFlow.swift) — a
pendingEdit still unconsumed by then means the closing didChangeText
never came, and the editor runs the same sync itself. Breadcrumb in
~/.edmund/logs: healing storage edit that bypassed didChangeText.
Never build a sync path on the assumption that didChangeText follows
every edit.
Round 6 corollary: a bypassed edit also leaves TextKit 2's private selection
fixup (_fixSelectionAfterChangeInCharacterRange:) queued; it fires at the
nextendEditing — even an attribute-only restyle — and leaps the caret
blocks away, moving even a freshly set valid caret. The heal sets the caret
before the sync and re-asserts it after (+EditFlow.swift). This class
does not reproduce headless; see the routing in §8.
4. TextKit 2 drawing model
All custom visuals are drawn by DecoratedTextLayoutFragment (custom
NSTextLayoutFragment,
Sources/EdmundCore/TextView/EditorTextView+TextKit2.swift:160), vended via
the layout-manager delegate. Two custom attribute keys (same file, lines
28/32):
Fragments tile vertically so a multi-line run reads as one continuous box/bar. A box's bottomPad grows the last fragment's own frame — TextKit 2 omits trailing paragraph spacing from the fragment, so padding done any other way is dead space.
.fragmentOverlay
Character
An image or stroked vector path at a character's laid-out position: rendered math, list bullets/checkboxes, callout header icon+name image, custom-title callout icon (path)
The anchor glyph is hidden (hiddenFont + clear color) and .kern reserves the drawing's advance width — the same trick the table renderer uses.
The image-wedge constraint (open, not solved). Drawing an image overlay
on a multi-line (wrapping) fragment re-triggers a layout pass that wedges
the fragment to one line. Drawing a shape (stroked CGPath) does not.
That is why the wrapping callout custom-title icon is a stroked path parsed
from vendored Lucide geometry, never an image. Any new overlay that could
share a line with wrapping text must be a shape, not an image. Full saga:
docs/investigations/archives/callout-title-wrap-investigation.md.
5. Read mode contract
Read mode is a separate WKWebView, not an editor styling mode
(Sources/EdmundCore/Export/). The contract: one parser, two back-ends —
the same swift-markdown Document the editor parses is walked by
SyntaxHighlighter.SpanCollector (→ editor attributes) and by HTMLRenderer
(→ HTML), themed from the sameEditorTheme via HTMLTheme, so the two
renderings cannot drift. When adding a feature, implement it in both
back-ends or document the divergence.
Hard properties of the web view (keep them):
JavaScript disabled; every asset inlined (math as high-DPI PNG data
URIs — SwiftMath has no SVG path; icons as inline Lucide SVG) so it needs
no file/network reach. Remote images off by default
(Sources/EdmundCore/Export/ReadRenderOptions.swift).
Inline HTML: only the whitelist SyntaxHighlighter.htmlFormatTags
(u/kbd/mark/sub/sup,
Sources/EdmundCore/Parsing/SyntaxHighlighter.swift:22) renders in either
mode; everything else stays escaped/color-only. A real <br> break would
need to mutate storage — forbidden by Invariant 1.
Export/Print run the same HTML through WKWebView.printOperation
(Sources/EdmundCore/Export/MarkdownPrinter.swift) for vector text.
6. Known weak points (open as of 2026-07-05)
State these plainly when designing near them; none is solved.
TextKit 2 height estimates are the root of most viewport glitches: an
off-screen fragment's frame (and the total document height) is an estimate
until layout reaches it — scroller jumps, scroll-to-target lands wrong (a
documented TK2 limitation; TextEdit shows it too). Mitigations in place,
not cures: documents ≤ fullLayoutMaxLength (100k UTF-16,
Sources/EdmundCore/TextView/EditorTextView.swift:80) are kept fully laid
out by scheduleFullLayoutSettle() wrapped in preservingViewportAnchor
(+LazyStyling.swift:121, +TypewriterScroll.swift:22);
repairContentAboveOrigin() (+LazyStyling.swift:151) fixes content
stranded above y=0; centerViewportOnCaret re-measures after its first
scroll. Never trust an off-screen fragment's y-coordinate without laying
out the span first.
The image-wedge constraint (§4) applies to every new overlay.
Open bugs (misc/backlog.md): callout at end-of-file renders an extra
un-prefixed line in the callout color (live incremental-restyle path, not
static rendering); footnotes don't render in either mode; attached images
create blank space below; math doesn't render in read mode (and has wrong
padding in edit mode); delete caret drift and viewport-estimate glitches
remain on the ongoing list.
Crash reporter endpoint is a placeholder:
CrashReporter.reportingEndpoint is https://REPLACE-ME.invalid/crash
(Sources/EdmundCore/Diagnostics/CrashReporter.swift:27) and the
Settings ▸ Advanced toggle is commented out
(Sources/edmd/Settings/AdvancedSettingsView.swift). Do not treat crash
uploading as live.
7. Before you design something new — checklist
Run this before proposing any new mechanism, subsystem, or refactor:
Invariant 1: does it insert/strip characters, use
NSTextAttachment, or mutate storage outside the
shouldChangeText→didChangeText path (or during IME composition)? If
yes, redesign as attributes/overlays.
Invariant 2: does it touch NSTextView.layoutManager or store
NSTextBlock/NSTextTable? If yes, stop.
Sync assumptions: does it assume didChangeText follows every
mutation, or that a set caret stays put across the next endEditing?
Both assumptions are proven false (§3).
Geometry: does it read an off-screen fragment frame, or restyle
without invalidateLayout(for:) when height changed? (§2, §6.)
Both back-ends: does a rendering feature cover Edit and Read
(§5)?
Prior art: check ARCHITECTURE.md §14 — especially
nodes-app/swift-markdown-engine,
an independent AppKit+TextKit 2 live-preview engine solving the same
problems — before inventing a new mechanism for an editing-experience
problem.
Weak points (§6): does the design lean on anything listed there?
Label it as such; unproven mitigations are "open/candidate", never
"fixed".
Verification plan: unit test if headless can repro; otherwise plan
a live repro (ReproScript) — do not ship a caret/IME/viewport fix on
reasoning alone. Visual claims are measured from screencapture
pixels, not eyeballed.
Process rules live in sibling skills, but never contradict them here: branch
per fix off main; never auto-push/PR/merge; swift test green + visual
verification before commit; never blanket pkill -x edmd (the maintainer's
daily-driver app shares the binary name — pgrep and kill only your own
PID); never request macOS Computer Access permissions.
When NOT to use this skill
You need…
Use instead
Whether/how to gate a change, commit discipline, scope control
edmund-change-control
A symptom → cause triage path for a bug you're seeing
edmund-debugging-playbook
The blow-by-blow history of a past investigation
edmund-failure-archaeology
TextKit 2 / AppKit theory beyond Edmund's specific contract
