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Left/Right Margins (DECLRMM, Mode 69)
DECLRMM (mode 69) enables left and right scroll margins, similar to top/bottom margins set by DECSTBM. When enabled, DECSLRM sets the left and right margins. Scrolling and editing operations are confined to the margin area.
How this is testedautomated
Enable DECLRMM (
Enable DECLRMM (
CSI ? 69 h), check getMode("leftRightMargin") returns true. Confirms the backend tracks left/right margin mode.The same probe runs against headless backends (via Termless) and real terminal apps (via a daemon launched in each terminal). This lets us distinguish parser correctness from rendering correctness.
Analysis2026-04-06
Supported by 9 of 14 backends (64%)
Terminal Applications
| Terminal | Version | Support | Notes |
|---|---|---|---|
| iTerm2 | 3.6.9 | ✓ yes | |
| Ghostty | 1.3.1 | ✓ yes | DECLRMM not tracked by ghostty-web WASM backend |
| VS Code | ✓ yes | ||
| Warp | ✓ yes | ||
| Cursor | ✓ yes | ||
| Terminal.app | ✓ yes |
Headless Backends
Parser correctness only — a ✓ means the parser accepts the sequence.
| Backend | Version | Support | Notes |
|---|---|---|---|
| vterm | 0.2.0 | ✓ yes | DECLRMM not implemented |
| Alacritty | 0.26.0 | ~ partial | DECLRMM not exposed via alacritty_terminal API |
| Kitty | 0.40.0 | ~ partial | DECLRMM not exposed via Python batch bridge |
| WezTerm | 0.1.0-fork.5 | ~ partial | DECLRMM not exposed via wezterm-term API |
| vt100.js | 0.2.1 | ✗ no | DECLRMM not implemented — pure TypeScript emulator |
| xterm.js | 5.5.0 | ✗ no | DECLRMM not implemented in xterm.js |