Demo — GUI Mandelbrot, multithreaded tiled zoom

• Type: feature
• Track: B
• Status: done
• Owner: trackE
• Opened: 2026-06-22
• Relation: Builds on feature-demo-mandelbrot (compute kernel + native-asm path) and the existing examples/mandelbrot/mandelbrot.pas (portable Double escape-time kernel + deterministic checksum oracle). GUI via lib/pcl (gtk3, glarea). Phase 3 leans on feature-parallel-processing / feature-async-language-surface.

Goal

An interactive GUI Mandelbrot with fast zooming on a tiled surface, where the headline feature is multithreaded rendering (use real cores so deep zooms stay responsive). Console vs GUI was open; GUI preferred (smooth interactive pan/zoom). The multithreading is the point of the ticket.

Phased plan

1. Mono-thread GUI. GTK3/GL window (lib/pcl), tiled framebuffer, interactive pan/zoom/reset, progressive/tiled redraw so a zoom paints coarse→fine. Reuse the existing portable Double escape-time kernel. Single render thread.
2. Multithread. Split the viewport into tiles, farm them to worker threads (work queue), recombine into the surface; redraw tiles as they finish. Prerequisite gap: the RTL has only cooperative concurrency (channel/coroutine/scheduler/syncobjs) — there is no OS-thread primitive (no PalThread/clone(CLONE_THREAD)). Real multicore needs a PAL thread-spawn (clone with shared VM + a per-thread stack) + join + a mutex. File/own that as a sub-task (PAL, like the recent process-spawn work) before this phase can use multiple cores.
3. Language-level parallel processing (big to-do). Re-express the tile loop with the language's parallel constructs instead of manual threads — depends on feature-parallel-processing / feature-async-language-surface maturing. Lowest priority; do phases 1–2 first.

Constraints

• NO automated tests without explicit permission. A multithreaded render (especially repeated in a test loop) can peg every core to 100% and lock the machine. This demo is manual-validation only unless the user explicitly asks for a smoke test — and any such test must be opt-in, bounded (tiny image, low iteration cap, single short run, thread count capped). Do NOT wire it into make lib-test/make demos as a runtime test; compile-smoke at most.
• Platonic source; GUI via lib/pcl; any math/runtime/compiler gaps found stay filed as separate tickets.
• The existing console examples/mandelbrot/mandelbrot.pas keeps its deterministic checksum oracle; this GUI/threaded demo is visual + interactive and is not a deterministic oracle.

Acceptance

• Phase 1: a usable GUI Mandelbrot with interactive pan/zoom and tiled redraw, compiles with $(PXX_STABLE), runs on the host (manual check).
• Phase 2: tiles rendered across N worker threads with a visible speedup on deep zooms; clean thread shutdown on quit; no automated stress test added without permission.
• Phase 3: the tile loop expressed via language parallel constructs (separate follow-up once those land).

Log

• 2026-06-22 — Opened on user suggestion: GUI, fast interactive tiled zoom, multithreaded as the core feature; phased mono → threads → language parallelism; explicit no-automated-multithread-tests-without-permission guardrail.
• 2026-07-20 — resolved, commit HEAD.