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Terminal renderer

Devolutions.Terminal.Render converts detached TerminalSnapshot instances into immutable row/run plans and paints those plans directly to Avalonia's Skia canvas.

Pipeline

  1. TerminalRenderPlanner resolves color, faint/inverse state, hyperlink boundaries, and terminal text clusters with their original cell coordinates.
  2. SkiaTerminalRenderer selects the configured face, then Cascadia Mono, Consolas, Segoe UI Emoji, and finally Skia's platform fallback.
  3. HarfBuzz shapes each cell cluster with its complete same-style run as context. Contextual scripts, combining text, and emoji ZWJ sequences retain shaping context, while every cluster origin remains fixed to the terminal grid.
  4. TerminalSkiaDrawOperation leases Avalonia's active canvas and draws cached Skia text blobs, decorations, overlays, and the cursor.

The renderer handles normal/bold/italic faces with synthetic bold or skew only when the selected family lacks the requested face. It supports faint, underline, strikethrough, hyperlink underline, CJK wide cells, color emoji, common Powerline separators, and all Windows Terminal cursor shapes.

Windows Terminal font sizes are point sizes, not raw Avalonia DIPs. The renderer uses the same 96/72 point-to-DIP conversion and nearest-pixel cell metric rounding as Atlas. The packaged Cascadia Mono regular and italic faces remain terminal-only resources; Fluent application chrome uses the platform Segoe UI family.

Cache and invalidation

The glyph cache is an LRU with a configurable hard capacity (4096 entries by default). Keys include text content, style, font size, and render scale. Cache entries own their SKTextBlob; eviction and renderer disposal release them deterministically. Typeface and paint resources are renderer-owned and are not shared with Avalonia's graphics device.

Resize clears shaped resources when render scale changes. Invalidate clears them after a font or graphics-resource change. No GPU surface or canvas is held across draw calls, so Avalonia device loss or backend recreation cannot leave a stale device object in the renderer.

TerminalFrameDiffer.GetDirtyRows is the renderer-neutral dirty-region contract. The control retains the previous frame and computes changed text and cursor rows for compositor integration. Dynamic selection, search, and hovered hyperlink ranges are separate overlays so they do not invalidate glyph entries.

Output-pump coalescing

The ConPTY read loop raises one engine invalidation per 16 KiB read. TermControl collapses queued invalidations into a single trailing UI-thread drain (DrainEngineInvalidation): a producer that finds the pending flag already set adds no new dispatcher post, and a drain that observes fresh output re-queues itself. The engine feed itself stays on the PTY thread so cursor-position report replies (CSI 6n) return before the application prints more. The drain is what fires the accessibility, scroll-mark, viewport, and IME-cursor notifications, so listener fan-out runs once per burst instead of once per chunk.

The cursor blink timer is damage-gated: a tick only invalidates when it can change pixels — the pane must be focused (unfocused panes draw a static cursor), the engine must report a blinking, visible cursor (DECSET 12/25), and the control must be effectively visible. Losing focus out of a mid-blink dark phase repaints once so the static cursor is solid.

tools/Devolutions.Terminal.Bench measures this path end to end (bench control --mb 8): a deterministic 8 MiB corpus is fed in 16 KiB chunks through a fake connection with dispatcher drains paced at 60 Hz, reporting engine invalidations vs posts vs drains alongside MB/s (medians over runs). Coalescing takes an 8 MiB burst from 514 drains to roughly one per frame.

Performance contract

Steady-state paint reuses Skia paints, cached text blobs, Powerline paths, and a cached shaping delegate. Indexed loops avoid interface-enumerator allocations. The immutable frame is the allocation boundary: changing terminal content allocates a new snapshot/plan, while repainting an unchanged frame does not allocate per cell.

Headless tests cover cache bounds and reuse, Unicode shaping, overlay ordering, cursor geometry, scale invalidation, dirty rows, and warm-render allocation.