Debugging Silvery

Canonical reference for debugging rendering issues. All other docs link here instead of duplicating.

Environment Variables

Verification Modes

# Buffer-level: incremental render phase must produce same buffer as fresh render
SILVERY_STRICT=1 bun run app

# ANSI-level: verify output via internal VT100 parser (fast, same process)
SILVERY_STRICT_TERMINAL=vt100 bun run app

# Terminal-level: verify via independent xterm.js emulator
SILVERY_STRICT_TERMINAL=xterm bun run app

# Terminal-level: verify via Ghostty WASM emulator
SILVERY_STRICT_TERMINAL=ghostty bun run app

# Multiple backends (comma-separated)
SILVERY_STRICT_TERMINAL=vt100,xterm bun run app

# All backends
SILVERY_STRICT_TERMINAL=all bun run app          # vt100 + xterm + ghostty

# Accumulated ANSI: replays ALL frames (O(N^2)) to catch compounding errors
SILVERY_STRICT_ACCUMULATE=1 bun run app

Notes:

• SILVERY_STRICT=1 → enables buffer-level check AND vt100 output verification
• SILVERY_STRICT_TERMINAL=all → shorthand for vt100,xterm,ghostty

Diagnostics

All diagnostic output is routed through loggily structured logging. Use DEBUG for log output and TRACE for span timing.

# All silvery diagnostic output (file-based to avoid stdout corruption)
DEBUG=silvery:* DEBUG_LOG=/tmp/silvery.log bun run app

# Render phase stats only (nodes visited/rendered/skipped per frame)
DEBUG=silvery:content DEBUG_LOG=/tmp/silvery.log bun run app

# Per-node trace entries (requires SILVERY_STRICT for trace collection)
DEBUG=silvery:content:trace DEBUG_LOG=/tmp/silvery.log bun run app

# Per-cell debug (which nodes cover a specific cell during incremental rendering)
SILVERY_CELL_DEBUG=77,85 DEBUG=silvery:content:cell DEBUG_LOG=/tmp/silvery.log bun run app

# Pipeline phase timing spans
TRACE=silvery:pipeline DEBUG_LOG=/tmp/silvery.log bun run app

# Measure phase debug (text measurement calls)
DEBUG=silvery:measure DEBUG_LOG=/tmp/silvery.log bun run app

# Instrumentation counters (enables stats collection, also exposed on globalThis)
SILVERY_INSTRUMENT=1 bun run app

Loggily Namespace Reference

Namespace	What
silvery:pipeline	Frame-level spans with per-phase timing
silvery:content	Render phase stats per frame (render/skip counts)
silvery:content:trace	Per-node trace entries (skip/render decisions)
silvery:content:cell	Per-cell debug (node coverage at target coords)
silvery:measure	Measure phase debug (text measurement calls)
@silvery/ag-react	React reconciler pipeline spans

Enriched STRICT Errors

When SILVERY_STRICT detects a mismatch, the IncrementalRenderMismatchError automatically captures:

• Render-phase stats (nodes visited/rendered/skipped, per-flag breakdown)
• Cell attribution (mismatch debug context)
• Dirty flags, scroll state, fast-path analysis

The scheduler auto-enables instrumentation for the STRICT comparison render. No need for separate SILVERY_INSTRUMENT or SILVERY_CELL_DEBUG runs when diagnosing STRICT failures.

What Each Mode Catches (and Misses)

Mode	Catches	Misses
STRICT	Render phase bugs (wrong dirty flag evaluation, skipped nodes, wrong region clearing, scroll tier errors)	Output phase bugs, terminal interpretation bugs
STRICT_TERMINAL=vt100	changesToAnsi bugs where our parser disagrees with our generator (style transitions, cursor arithmetic)	Bugs where parser and generator agree but real terminals disagree (pending-wrap, \x1b[K in wrap state)
STRICT_TERMINAL=xterm	Terminal interpretation bugs (xterm.js-specific: OSC 66, wide char cursor, buffer overflow)	Ghostty-specific bugs, bugs requiring accumulated state
STRICT_TERMINAL=ghostty	Ghostty-specific terminal interpretation bugs	xterm.js-specific bugs
STRICT_ACCUMULATE	Compounding errors across multiple frames	Same limitation as vt100 (self-referential parser)

Hierarchy: STRICT (buffer) → STRICT_TERMINAL=vt100 (ANSI) → STRICT_TERMINAL=xterm (terminal) → STRICT_TERMINAL=all (cross-backend).

CI strategy:

• PR CI: SILVERY_STRICT_TERMINAL=vt100 (fast, zero deps)
• Nightly: SILVERY_STRICT_TERMINAL=xterm (independent emulator)
• Scheduled/allow-fail: SILVERY_STRICT_TERMINAL=ghostty (WASM, has known grapheme bugs)
• Local debug: SILVERY_STRICT_TERMINAL=all

Diagnostic Workflow

1. Start with STRICT: SILVERY_STRICT=1 bun vitest run ... catches any incremental vs fresh render divergence immediately.

2. Write a failing test: If fuzz found it, extract the seed. If user-reported, construct a withDiagnostics(createBoardDriver(...)) test with minimal reproduction steps.

3. Read the mismatch error: The enhanced error includes cell values, node path, dirty flags, scroll context, and fast-path analysis. This tells you exactly which node diverged and why it was skipped.

4. Check instrumentation: SILVERY_INSTRUMENT=1 enables stats collection. View with DEBUG=silvery:content DEBUG_LOG=/tmp/silvery.log (loggily output) or programmatically via globalThis.__silvery_content_detail. Useful for understanding whether too many or too few nodes rendered.

5. Check the five critical formulas: layoutChanged, contentAreaAffected, contentRegionCleared, skipBgFill, childrenNeedFreshRender in renderNodeToBuffer (render-phase.ts). If any is wrong, the cascade propagates errors to the entire subtree.

6. Text bg inheritance: Text nodes inherit bg via inheritedBg (from findInheritedBg), not buffer reads. Viewport clears and region clears still affect buffer state, which matters for the getCellBg legacy fallback (used by scroll indicators). If your fix clears a region, verify it clears to the correct bg (usually null to match fresh render state).

Symptom → Check Cross-Reference

Symptom	Check First
Stale background color persists	bgDirty flag; inheritedBg from findInheritedBg; is region being cleared?
Border artifacts after color change	stylePropsDirty vs contentAreaAffected distinction; border-only change should NOT cascade
Scroll glitch (content jumps/disappears)	Scroll tier selection; Tier 1 unsafe with sticky; Tier 3 needs stickyForceRefresh
Children blank after parent changes	childrenNeedFreshRender → childHasPrev=false; is viewport clear setting childHasPrev correctly?
Absolute child disappears	Two-pass rendering order; absolute children need ancestorCleared=false in second pass
Content correct initially, wrong after navigation	Incremental rendering bug; SILVERY_STRICT=1 will catch it
Colors wrong but characters correct (garble)	Output phase: diffBuffers row pre-check skipping true-color Map diffs; check rowExtrasEquals
Text bg different from parent Box bg	inheritedBg from findInheritedBg; check if ancestor Box has backgroundColor; check region clearing
Flickering on every render	Check layoutChangedThisFrame flag; verify syncPrevLayout runs at end of render phase
Stale overlay pixels after shrink (black area)	clearExcessArea not called; check contentRegionCleared + forceRepaint interaction
CJK/wide char garble, text shifts right	bufferToAnsi cursor drift: wide char without continuation at col+1. Run SILVERY_STRICT_TERMINAL=xterm
Flag emoji garble at wide terminals (200+ cols)	bufferToAnsi/changesToAnsi cursor re-sync after wide chars; wrapTextSizing
Stale chars in ancestor border/padding after child shrinks	Descendant overflow: clearExcessArea clips to immediate parent. Use hasDescendantOverflowChanged() for recursive detection