[0101]

Problem

Treemon was pinned to net9.0 across all five projects. The spec recorded why:

net9.0 (not net10.0): Fable 4.28.0 FCS hangs with .NET 10 preview SDK

That blocker is real and reproducible — but the fix is not a simple TFM bump. Moving the client to net10.0 walks into a chain of three separate toolchain incompatibilities, each of which silently breaks something different.

Changes

Baseline

• All five projects (Shared, Client, Server, Cli, Tests) -> net10.0
• global.json -> SDK 10.0.110, plus allowPrerelease: false. With rollForward: latestMajor and the default allowPrerelease: true, dotnet silently selected an installed 10.0.400-preview SDK — exactly the preview SDK the original spec note blamed.
• .github/workflows/ci.yml -> dotnet-version: 10.0.x
• Microsoft.Data.Sqlite 9.0.11 -> 10.0.10

The Fable version squeeze

There is exactly one viable Fable release. Each candidate was verified empirically, not assumed:

Version	Outcome
4.28.0 / 4.29.0	Deadlocks when the compiled project targets net10.0. Confirmed locally: the Fable process burned ~5s CPU over 10 minutes at a flat 59 MB — a hang, not slow compilation. Upstream fable-compiler/Fable#4326, closed with "use Fable 5". No fix will land on fable4.
5.1.0+	Fable#4529 made option_type() carry None/Some union cases, so FSharpType.IsUnion(typeof<'a option>) now returns true. Fable.SimpleJson matches its UnionType pattern before OptionType, so every option field in a remoting response fails to deserialize. Upstream Fable.Remoting#394 (open); the two-line fix is Fable.SimpleJson#111 (open, unmerged).
5.5.0+	Does the same to list_type (Fable#4698), and adds a hard error for inline functions referencing private values (Fable#4701) that rejects Fable.Remoting.MsgPack's write64bitNumber. Tracked as Fable.Remoting#396 (open, no maintainer response).
5.0.0	Predates all three. Stock Fable.Remoting works with no vendored patches.

So dotnet-tools.json pins fable to 5.0.0 (rollForward: false was already set). The reasoning is recorded in docs/spec/worktree-monitor.md with a revisit note.

This is the failure mode that mattered most: with Fable 5.4.0 the solution built, Fable compiled, Vite bundled, and the server started — but the dashboard rendered only skeleton placeholders. The API returned HTTP 200 with valid JSON, and there were zero console errors, zero page errors, and zero unhandled rejections. The exception was being swallowed into Elmish's DataFailed arm.

Feliz 3 migration

Fable 5 requires Feliz 3 — the Feliz 2.x compiler plugin targets the Fable 4 AST and throws Method not found: Fable.AST.Fable.Ident..ctor(...). Feliz 2.* -> 3.*, with the API changes it forces:

• React.fragment -> React.Fragment (App.fs, CanvasPane.fs, CardViews.fs, Components.fs, OverviewBand.fs)
• Interop.createElement -> HtmlHelper.createElement (CardViews.fs)
• React.createDisposable removed -> plain unit -> unit cleanup lambdas (Components.fs, OverviewChart.fs). This also resolved a useEffect overload ambiguity. Note ignore does not typecheck here (it is 'a -> unit), hence fun () -> ().
• svg.viewBox now infers the string overload rather than the (int, int, int, int) tuple one, so the icon helper in CardViews.fs takes an annotated viewBox: string and call sites pass "0 0 24 24" / "0 0 18 18".

Housekeeping

• .gitignore: *.fable-temp.csproj (generated by Fable on every run)
• AGENTS.md and review/rules/modern-fsharp-syntax.md: the "Fable 4.28 supports this syntax" notes now read "Fable 5.0"

Tests

dotnet build treemon.slnx — succeeds, 0 errors. dotnet fable and npm run build both succeed (122 modules, 453 kB bundle).

Suite	Result
Full (dotnet test)	2011 passed, 1 failed, 10 skipped (2022 total, 10m13s)
Category=Fast	1671 passed, 0 failed
Category=E2E	446 passed, 1 failed, 1 skipped (448 total, 5m43s)

The single failure is warm 250-path summary appears within one second (DiffViewerTests.fs), a server-side perf assertion with a 1000 ms budget. It is pre-existing, not a regression — I stashed this branch and measured unmodified main, which fails the same assertion worse:

	responseMs / displayMs
main (net9, Fable 4.28)	1073/1141, 1728/1837, 1618/1686
this branch (net10, Fable 5.0)	1183/1246, 1127/1196, 1163/1232

.NET 10 is in fact slightly faster on this path. The budget is simply too tight for this machine, so I left the threshold alone rather than silently relaxing a perf guard.

Runtime behaviour was verified separately with a Playwright probe against a server + Vite pair on isolated ports (60777/60778/60779) — the dashboard renders live data (11 cards, 11 branch names) with no console errors. Production and port 5000 were never touched.

Notes

• NU1903 for SQLitePCLRaw.lib.e_sqlite3 2.1.11 appears in the build. It is pre-existing: the advisory range is <= 2.1.11, and Microsoft.Data.Sqlite 9.0.11 already pulled the equally-affected 2.1.10. Left alone as unrelated scope.
• Fable should be un-pinned once Fable.SimpleJson ships the option/list ordering fix and Fable.Remoting addresses the inline private error. Both upstream issues are linked above and in the spec.

[Copilot]

Pull request overview

Migrates Treemon to .NET 10 and the compatible Fable/Feliz toolchain.

Changes:

• Targets .NET 10 across all projects and CI.
• Pins Fable 5.0.0, upgrades Feliz 3, and updates affected client APIs.
• Updates SQLite, documentation, and generated-file exclusions.

Show a summary per file

File	Description
.github/workflows/ci.yml	Uses .NET 10 in CI.
.gitignore	Ignores Fable temporary projects.
AGENTS.md	Updates Fable compatibility guidance.
docs/spec/worktree-monitor.md	Documents the toolchain constraints.
dotnet-tools.json	Pins Fable 5.0.0.
global.json	Selects a stable .NET 10 SDK.
review/rules/modern-fsharp-syntax.md	Updates Fable syntax guidance.
src/Cli/Cli.fsproj	Targets .NET 10.
src/Client/App.fs	Migrates fragment API.
src/Client/CanvasPane.fs	Migrates fragment API.
src/Client/CardViews.fs	Migrates element, fragment, and SVG APIs.
src/Client/Client.fsproj	Targets .NET 10 and Feliz 3.
src/Client/Components.fs	Migrates fragments and effect cleanup.
src/Client/OverviewBand.fs	Migrates fragment API.
src/Client/OverviewChart.fs	Migrates effect cleanup.
src/Server/Server.fsproj	Targets .NET 10 and updates SQLite.
src/Shared/Shared.fsproj	Targets .NET 10.
src/Tests/Tests.fsproj	Runs tests on .NET 10.

Review details

• Files reviewed: 17/18 changed files
• Comments generated: 1
• Review effort level: Medium