Add Windows WebView2 backend, port jsaddle-webkitgtk to GTK4/WebKitGTK 6.0

[hamishmack]

Summary

This PR adds a Windows backend, modernises the Linux backend, and makes the whole repo solve on every platform from one flat cabal.project.

• New package jsaddle-webview2 — Windows backend hosting the app in a native Win32 window with a Microsoft Edge WebView2 control.
• jsaddle-webkitgtk ported to GTK4 + WebKitGTK 6.0 — replaces the WebKit1/GTK3 code (and supersedes jsaddle-webkit2gtk, whose webkit2gtk-4.0 dependency no longer exists in nixpkgs).
• cabal.project restructured — flat unconditional packages: list with per-package buildable: False platform guards.
• Flake: haskell.nix bump + headless runtime smoke test for the WebKitGTK backend.

jsaddle-webview2

Mirrors jsaddle-wkwebview's design: async batches are evaluated as runJSaddleBatch(...) and results come back via chrome.webview.postMessage; synchronous results use window.prompt("JSaddleSync", …) intercepted natively in ScriptDialogOpening.

The C shim (cbits/WebView2Shim.c) is plain-C COM against the official WebView2.h — only the header is needed at compile time (fetched from the WebView2 NuGet package in nix/hix.nix for cross builds); WebView2Loader.dll is loaded dynamically at run time and the WebView2 runtime ships with Windows 11. WebView2 is single-threaded (STA), so all ICoreWebView2* calls are marshalled to the UI thread via PostMessage.

Cross-compiles from nix via hydraJobs.<system>.x86_64-w64-mingw32 (lib + demo exe verified). Not yet run on real Windows hardware — the prompt-bridge length limits and first-navigation timing are the known risks to check there.

jsaddle-webkitgtk (GTK4 / WebKitGTK 6.0)

Rewritten against gi-gtk4 / gi-webkit 6.x / gi-javascriptcore6: GLib main loop instead of gtk_main, windowSetChild/scrolledWindowSetChild, webViewEvaluateJavascript, and the WebKit 6 script-message + script-dialog (JSaddleSync prompt) bridge. Requires haskell.nix master (the WebKitGTK 6.0 pkg-config map entries — webkitgtk-6.0, javascriptcoregtk-6.0 — landed there recently); the flake input is bumped accordingly.

A new demo executable has a --smoke mode that asserts a full bridge round trip (async eval, plus a JS→Haskell callback through the synchronous prompt path) and exits 0/1.

Per-platform packaging

cabal.project if os(...) conditionals do not apply to packages: fields (silently ignored), so platform gating now lives in each .cabal file as buildable: False guards covering every component; the solver ignores dependencies of non-buildable components. Notes:

• jsaddle-terminal's demo moved behind a manual demo flag (default off): reflex-dom-core caps base below new GHCs and otherwise makes the whole project unsolvable.
• jsaddle-webkit2gtk stays out of the project: a missing pkg-config package (webkit2gtk-4.0) fails the solve regardless of buildable: guards.

CI / testing

• Plans verified for x86_64-linux, x86_64-w64-mingw32 cross, javascript-unknown-ghcjs cross, and aarch64-darwin — each contains exactly the intended components.

• New flake check checks.<linux>.webkitgtk-smoke (also in hydraJobs.checks) runs the WebKitGTK demo's --smoke mode headlessly under Xvfb inside the nix sandbox (dbus config from the store, WebKit's bubblewrap sandbox off, llvmpipe software EGL) and passes iff the bridge round-trips:

  SMOKE eval 6*7 = 42.0
  SMOKE callback got 123.0
  SMOKE-OK
  

[Copilot]

Pull request overview

Adds a new Windows-native JSaddle backend via Microsoft Edge WebView2, modernizes the Linux WebKitGTK backend to GTK4/WebKitGTK 6.0, and restructures packaging/Nix so the repo can solve and check consistently across platforms.

Changes:

• Introduces jsaddle-webview2 (Win32 + WebView2) with a C shim and Haskell transport mirroring jsaddle-wkwebview.
• Ports jsaddle-webkitgtk to GTK4 + WebKitGTK 6.0 and adds a headless --smoke demo mode plus a Nix/flake check to run it under Xvfb.
• Makes multi-platform solving work from a flat cabal.project by pushing platform gating into per-package buildable: False guards.

Reviewed changes

Copilot reviewed 17 out of 19 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
nix/webkitgtk-smoke.nix	Adds a headless WebKitGTK runtime smoke test derivation (Xvfb + dbus-run-session + llvmpipe).
nix/hix.nix	Bumps compiler, adds WebView2 header fetch for cross builds, and expands cross platforms.
jsaddle-wkwebview/jsaddle-wkwebview.cabal	Tightens native buildability to macOS/iOS (with ghcjs/javascript stubs elsewhere).
jsaddle-webview2/src/Language/Javascript/JSaddle/WebView2/Internal.hs	Implements the JSaddle transport wiring for WebView2 (async + prompt-based sync bridge).
jsaddle-webview2/src/Language/Javascript/JSaddle/WebView2.hs	Public API for running JSaddle apps in a WebView2-backed native window.
jsaddle-webview2/README.md	Documents how WebView2 backend works and how to build/run it.
jsaddle-webview2/LICENSE	Adds MIT license file for the new package.
jsaddle-webview2/jsaddle-webview2.cabal	Defines the new Windows-only package + demo component.
jsaddle-webview2/demo/Main.hs	Adds a minimal demo app (counter + ticker).
jsaddle-webview2/cbits/WebView2Shim.c	Provides the Win32/WebView2 hosting shim, message-loop marshalling, and JS bridge integration.
jsaddle-webkitgtk/src/Language/Javascript/JSaddle/WebKitGTK.hs	Ports backend to GTK4/WebKitGTK 6.0 and updates the bridge mechanism.
jsaddle-webkitgtk/jsaddle-webkitgtk.cabal	Updates dependencies and platform gating; adds a Linux-only demo executable.
jsaddle-webkitgtk/demo/Main.hs	Adds demo with --smoke mode for CI headless runtime verification.
jsaddle-webkit2gtk/jsaddle-webkit2gtk.cabal	Adds explicit Linux-only native buildability guard (plus ghcjs/javascript stubs).
jsaddle-terminal/jsaddle-terminal.cabal	Adds demo flag gating and marks Windows as non-buildable for POSIX raw-mode terminal code.
flake.nix	Wires the new webkitgtk-smoke check into checks and hydraJobs on Linux.
flake.lock	Updates flake inputs (notably haskell.nix / nixpkgs pins) to support newer GTK/WebKit mapping.
cabal.project	Flattens unconditional package list and documents why per-package buildable guards are used.
.gitignore	Ignores Nix result* outputs.

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