[Marc-André Moreau (mamoreau-devolutions)]

Summary

Completes 5 iterative rounds of Sixel and Kitty graphics/keyboard protocol bug fixes and performance optimizations across Devolutions.Terminal.Core, Devolutions.Terminal.Render, Devolutions.Terminal.Control, Devolutions.Terminal.Automation, and docs/advanced-vt-protocols.md (50 bug fixes + 50 performance improvements).

Highlights by Round

Round 1 — Parser & Sixel Decoder

• Bug fixes (1–10): Ignore C0 cursor/scroll controls during DCS passthrough; ignore DEL (0x7F) in APC; prevent BEL from terminating StringIgnore on oversized APC strings and handle non-\ escapes in StringEscape; ignore whitespace/C0 inside Sixel !Pn<sixel> and parameter lists; preserve parameter indices on leading ;; default omitted DECGRA Pad to 1 and ignore mid-stream " mutations; charge _remainingPixelWrites on '?' repeats to prevent CPU DoS; wrap color register indices modulo 256 and HLS hues modulo 360°; resolve WrapPending and report post-scroll AnchorRow in DispatchSixel.
• Performance (1–10): Vectorized SearchValues<byte> bulk scanning for DCS and APC payloads; zero-allocation TerminateDcs span dispatch and pooled FinishApc UTF-8 decoding; static DefaultPalette, stackalloc parameter parsing, BitOperations.PopCount, fast 1:1 pixel writes, ArrayPool<ushort> Sixel working buffer with single-pass background fill, DECGRA pre-sizing, and PercentToByteTable LUT.

Round 2 — Kitty Graphics Decoder, Chunking & Store

• Bug fixes (11–20): Carry-buffered non-4-char-aligned m=1/m=0 chunked base64 reassembly; pre-scan i=, I=, and q= on parse failures; inherit q= and I= across continuation chunk errors; abort incomplete chunked transfers when non-continuation actions (a=d, a=p) arrive; validate and resolve I=<uint32> image numbers; replace existing non-zero (imageId, placementId) overlays on active buffer; reference-count shared KittyImageData bytes across placements; validate o=, C=, m=, and q=; support d=i/d=I with p= and free unreferenced data on d=P; scope d=a/d=A to the active buffer.
• Performance (11–20): Zero-string Convert.TryFromBase64Chars; direct streaming base64 quantum decode into _kittyChunkPayload; ArrayPool<byte> inflate scratch buffer; exact-size raw f=24/f=32 zlib inflate without MemoryStream; zero-copy f=32 ownership transfer; Vector128.Shuffle SIMD RGB24 -> RGBA32 expansion; zero-alloc comma key scanning and inline integer parsers; struct KittyContinuationChunk fast path; Utf8.TryWrite stackalloc APC responses; O(1) LinkedList<uint> store eviction.

Round 3 — Skia Renderer, Planner & Frame Differ

• Bug fixes (21–30): Compare CellGeometry, image data identity, and all KittyImage placement fields in TerminalFrameDiffer; support partial Kitty crops (X/Y/w/h individually); render nothing on out-of-bounds CropX/CropY; clip viewport-overflowing Sixel/Kitty overlays instead of squishing aspect ratio; cull off-screen scrollback images before DecodeImage; scale natural Kitty dimensions and pixel offsets by PhysicalPixel on HiDPI displays; clamp PixelOffsetX/PixelOffsetY within a single cell; sort negative-z placements by (ZIndex, Id); fix Dictionary.First() cache eviction with true LinkedList LRU; clear cached images and _invalidImages in Invalidate().
• Performance (21–30): Eliminate per-frame LINQ/HashSet/closure allocations in DrawImages; direct GetPixelSpan() Sixel and Kitty RGBA blits without SKColor[] LOH allocations; span-based DecodeCodecImage; shared SKBitmap cache keyed by underlying pixel-data identity (ReferenceEqualityComparer); early exit before Save/ClipRect/Restore when empty; skip sort when monotonic; guard stale-cache sweep; zero-alloc FilterActiveBufferImages in TerminalRenderPlanner; fast TerminalFrameDiffer checks.

Round 4 — Kitty Keyboard Protocol & Buffer Image Anchors

• Bug fixes (31–40): Separate primary and alternate screen Kitty keyboard flag stacks; reset flags to None when popping past bottom and evict oldest entry on full 16-entry stack; validate CSI = mode (1/2/3); add Kitty functional key codepoints (F13–F24, keypad, lock/system, and modifier keys); colon-join multi-rune associated text; lowercase non-ASCII Unicode base codepoints; emit SS3 P/Q/S for unmodified F1/F2/F4; gate EncodeKittyTextInput on ReportAllKeysAsEscapeCodes and suppress duplicate OnTextInput on modified Kitty keys; track (Key, PhysicalKey) in ProcessKeyUpInput to suppress orphan releases; validate raw s/v dimensions before zlib decompression.
• Performance (31–40): ImageAnchorLayoutVersion O(1) gate on PruneEvictedImages; viewport-first scan + monotonic O(log N) binary search in TryResolveImageAnchor; zero-LINQ ExtractBufferAnchors on resize; min/max LogicalLineId fast skip in Reflow/LastTextLine; reuse pre-resolved anchors in CreateSnapshotImages; stackalloc ReapUnplacedKittyImages; closure-free RemoveKittyOverlays; span-hoisted SixelImage.ToRgba32; layout-version sync in AddImage; zero-LINQ/zero-Enum.ToString() CreateObservedImages in Automation.

Round 5 — Sixel/Kitty Lifecycle, Screen Erase & Blitter Optimizations

• Bug fixes (41–50): Implement XTSMGRAPHICS (CSI ? Pi;Pa;Pv S); implement DECSET/DECRST/DECRQM 1070 (private Sixel palette) and 8452 (Sixel cursor-right mode); avoid advancing auto Kitty image ID on a=q queries or rejected payloads; preserve primary a=t images across \e[?1049h alternate-screen entry; clear erased viewport overlays on CSI 0/1/2 J and prune scrollback on CSI 3 J; resolve WrapPending and aspect-scaled cursor rows (c > 0, r == 0) on Kitty a=T; validate unsigned Kitty control values and allow tall/wide r/c placements beyond current viewport size; use SKAlphaType.Opaque for opaque codec images (e.g. JPEG); honor DECOM in DECSDM; clamp non-reflowed alternate-buffer anchors on narrowing.
• Performance (41–50): Single-pass stackalloc formatting + cached literals in KeyMapper.EncodeKitty, EncodeLegacyFunctionalKey, and EncodeKittyTextInput; boxing-free IsTextInputCandidate; pre-built CSI ? u byte response table; consecutive identical sixel byte run coalescing; fast '?' skip and contiguous '~' fill in WriteSixel; Utf8.TryWrite stackalloc XTSMGRAPHICS responses; SKAlphaType.Opaque tagging for opaque Sixel and f=24 Kitty bitmaps; branchless 257-entry stackalloc Sixel palette LUT in Skia; bounded TextBuffer.AdvanceLines batching.

Validation

• dotnet build Devolutions.Terminal.slnx -c Debug (0 warnings, 0 errors)
• dotnet build Devolutions.Terminal.slnx -c Release (0 warnings, 0 errors)
• dotnet test Devolutions.Terminal.slnx -c Release — 3,360 passed, 0 failed across all test suites (Core, Render, Control, Automation, Connection, App, UI, Settings, Settings.Editor, Cli, Package, Broker, Compatibility)