WebView UI

Use this skill when working on Pulp's optional WebView layer: HTML/JS panels, Monaco-style tools, documentation views, or mixed native/WebView windows.

Core Positioning

Pulp WebView is an opt-in compatibility layer, not the primary UI path. Default to native Pulp UI unless the task truly benefits from existing web content.

Core Runtime Pieces

Main APIs:

• pulp::view::WebViewPanel
• pulp::view::WebViewOptions
• pulp::view::WindowHost
• pulp::view::AssetManager

Bridge contract:

• window.pulp.postMessage(type, payload, id)
• window.pulp.on(type, callback)
• native set_message_handler(...)
• native post_message(...)
• async evaluate_js(..., callback)

Platform Truth

PULP_BUILD_WEBVIEW=ON is now the honest opt-in switch for the common Pulp WebView layer on:

• macOS via WebKit
• Windows via CHOC's WebView2 path
• Ubuntu/Linux via CHOC's GTK/WebKitGTK path

Required platform notes:

• macOS: no extra package discovery beyond the system WebKit framework
• Windows: normal Visual Studio/CMake toolchain plus a WebView2-capable runtime
• Linux: gtk+-3.0 and webkit2gtk-4.1 available through pkg-config

Keep claims narrow:

• macOS has the deepest runtime proof today, including Monaco in a native host
• Windows and Ubuntu now have truthful opt-in configure/build/test proof plus pulp-webview-palette build proof
• do not describe that as full runtime parity unless the host-level proof is actually there

File drag-and-drop (from choc::ui::WebView)

CHOC's WebView layer carries file drag-and-drop, exposed to page JS — no Pulp C++ wiring needed, it rides the choc/gui/choc_WebView.h we already wrap in core/view/src/web_view.cpp:

• Drag a file OUT of the webview: call window.chocStartFileDrag(["/abs/path", ...]) from a pointerdown/mousedown handler.
• Drop files INTO the webview: listen for the chocfilesdropped event; e.detail.paths is the array of absolute paths.

Platform reality: macOS (WKWebView) + Linux (WebKitGTK) — both directions. Windows (WebView2) — inbound only (outbound is a documented no-op: windowed WebView2 can't start a drag from web content without moving to composition hosting). The JS call exists on all platforms for parity.

⚠ Interim source: this lands via an interim fork pin of CHOC (danielraffel/choc tag pulp-webview-dnd-poc1, carrying WebView drag-and-drop support; Tracktion/choc pull request 64 is the upstream tracker). When that support is available upstream, the pin reverts to Tracktion/choc with no API change. The runnable reference lives in the choc repo (examples/webview_drag_and_drop.cpp), not in Pulp.

Choose A Loading Mode

1. Simple inline proof

• use set_html(...)
• append make_webview_bridge_bootstrap_script()
• good for the smallest lifecycle/message tests

1.5. Placeholder first paint

• set WebViewOptions::initial_html when the final page is dark-themed and the platform WebView would otherwise flash white before the real content loads
• keep it lightweight and self-contained so it paints immediately
• combine with transparent_background = true when the final page also wants a transparent/native host background

2. Embedded bundled assets

• use pulp_add_binary_data(...)
• register generated arrays with AssetManager
• serve through make_webview_embedded_resource_fetcher(...)
• this is the truthful offline/distributable path

3. Directory-backed dev assets

• use make_webview_directory_resource_fetcher(...)
• use this for local framework payloads that already exist on disk
• good for Monaco / richer web tooling during development

Monaco Guidance

Do not chase Monaco's deprecated AMD path.

Use a prebundled ESM workflow:

• bundle one app entry
• bundle the worker files
• point MonacoEnvironment.getWorkerUrl(...) or getWorker(...) at those generated worker files
• expose window.monaco only if the page wants that global convenience

Treat raw Monaco source or raw ESM output as build inputs, not as the final browser payload. Worker and CSS handling belongs to the bundler.

Current proven local recipe:

cd /Users/danielraffel/Code/monaco-editor
npm install
npm run build-monaco-editor

cd /path/to/pulp
node examples/webview-monaco/build_monaco_bundle.mjs \
  --monaco-root /Users/danielraffel/Code/monaco-editor \
  --out-dir /path/to/pulp/build/examples/webview-monaco/dist

cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug -DPULP_BUILD_WEBVIEW=ON \
  -DPULP_MONACO_BUNDLE_DIR=/path/to/pulp/build/examples/webview-monaco/dist
cmake --build build --target pulp-webview-monaco -j8
./build/examples/webview-monaco/pulp-webview-monaco

Important notes from the working proof:

• use the local build script in examples/webview-monaco/build_monaco_bundle.mjs
• prefer editor.main.js so Monaco's real CSS/features come through
• worker URLs must point at the built vs/languages/features/... layout
• the current Monaco build imports @vscode/monaco-lsp-client; for the Monaco example we stub that at bundle time because the example is not doing LSP
• browser preview is useful for cheap layout proof, but bridge unavailable is expected there because only the native host injects window.pulp
• the native proof should reach editor ready

Window Embedding

WebView backend availability and host embedding availability are separate. A WebViewPanel can exist even when the surrounding WindowHost or PluginViewHost cannot accept a native child surface. Always check:

• the panel's native_handle()
• the host's native content/view handle where applicable
• the boolean returned by attach_native_child_view(...)

Current host truth:

• standalone WindowHost child embedding is built in on macOS
• built-in iOS WindowHost does not expose standalone child-view embedding handles yet
• PluginViewHost child embedding is built in on macOS and iOS
• Windows/Linux/Android embedding is factory-backed through WindowHost::Factory or PluginViewHost::Factory

For palette / inspector style UI:

• create the parent WindowHost
• create the child/palette WindowHost
• use attach_native_child_view(...)
• resize with set_native_child_view_bounds(...)
• detach on close

For standalone native-child embeds, do not size from startup constants once the host is live. Read WindowHost::get_content_size() for the current content bounds, attach the child with that size, and keep it in sync via WindowHost::set_resize_callback(...). examples/webview-monaco/main.cpp is the current reference pattern.

For plugin-editor embedding, use the view::NativeViewHost widget — do NOT hand-roll plugin_view_host()->attach_native_child_view(...):

• add a NativeViewHost to the editor tree, size it with flex (host->flex().flex_grow = 1.0f to fill), and call set_native_child(panel->native_handle(), snapshot_fn)
• the widget drives attach / bounds / clip / detach itself from its host back-reference + Yoga layout, so it works under BOTH a PluginViewHost (editor) and a WindowHost (standalone). It falls back from plugin_view_host() to window_host() (core/view/src/native_view_host.cpp try_attach() / active_viewport_transform())
• it also honors the active design-viewport transform and routes headless snapshot capture for free — no on_view_opened/resized/closed plumbing
• see examples/webview-plugin/ for the reference Processor-backed editor

Native-editor gotchas (G2, learned building examples/webview-plugin)

• Retention: own the WebViewPanel on the Processor, never in the view tree. ViewBridge::close() destroys the whole view tree on editor close (core/format/src/view_bridge.cpp). A view-owned WebViewPanel is recreated every close/reopen, discarding the WKWebView's JS heap + page/scroll state. Put the panel on the Processor (create once, lazily in create_view()); the NativeViewHost only BORROWS native_handle(). Rule of thumb: anything that must survive editor close lives on the Processor. Pinned by a pointer-identity test across two open/close cycles in test/test_webview_plugin_example.cpp.
• The old plugin_view_host()-only attach SILENTLY no-ops in standalone. In standalone the root is hosted by a WindowHost, not a PluginViewHost (core/format/src/standalone.cpp), so plugin_view_host() is null and a hand-rolled attach returns without attaching — the WebView never appears and nothing errors. NativeViewHost's window_host() fallback is the fix.
• WKWebView frame scaling REFLOWS, it does not zoom. CSS px track the frame, so a letterbox-scaled native child does not visually match a scaled Pulp sibling. WKWebView.pageZoom = scale parity is deliberately DEFERRED (not a native-editor blocker).
• Headless snapshot_png() returns ZERO bytes even when is_ready()==true — WKWebView's takeSnapshot needs an on-screen window with painted content. Example/headless tests therefore assert the snapshot ROUTE by equality (NativeViewHost::capture_native_overlay_png == panel->snapshot_png()), not a literal non-empty size; the non-empty forwarding guarantee is pinned at the widget level in test/test_native_view_host.cpp.

For a standalone app that should behave like a single-pane WebView shell:

• use pulp::format::StandaloneApp::run_with_editor(...)
• set StandaloneConfig::show_settings_tab = false to skip the outer standalone Editor/Settings chrome entirely
• examples/webview-plugin/main.cpp is the current kiosk-style proof
• if the page is dark on first launch, seed WebViewOptions::initial_html with a dark placeholder so the host opens into the final visual family
• if you want the standalone bundle to ship a custom app icon, keep that in the example CMakeLists.txt via pulp_app_icon(...); it stays optional and should not be wired through the runtime WebView code itself

Validation Loop

For focused local proof:

cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug -DPULP_BUILD_WEBVIEW=ON
cmake --build build --target pulp-test-webview -j8
./build/test/pulp-test-webview --reporter compact

If example code changed, also build the relevant example target:

cmake --build build --target pulp-webview-palette -j8

Current proof expectations:

• encode/decode helpers pass
• bridge helpers pass
• resource fetchers pass
• example builds
• Windows/Ubuntu opt-in builds can configure and run the focused test slice
• live callback-ready cases may skip in headless or limited environments

Screenshot / Preview Workflow

For quick visual inspection, keep a static browser preview page beside the runtime assets when useful. That gives you a cheap screenshot path without pretending the browser preview replaces native validation.

For Monaco specifically:

• browser-served proof page is useful to confirm bundling, workers, and CSS
• native-host proof is still required before claiming the bridge is working

Headless capture of a native WebView overlay

A WebView is an OS-composited native child view (attach_native_child_view); it is NOT painted into the Pulp Skia canvas, so a plain headless capture (render_to_png / a host back-buffer read) of an editor that hosts one sees a blank where the WebView is. To make a WebView editor self-verifiable headlessly, the owning Pulp View wrapper must opt in:

1. call set_contains_native_overlay(true) when it attaches the WebView, and
2. override capture_native_overlay_png() to forward WebViewPanel::snapshot_png() (macOS: WKWebView takeSnapshot, no screen-recording permission needed).

The smart capture path (pulp::view::capture_view / pulp-screenshot --backend auto / standalone --screenshot) then returns the in-process WebView snapshot instead of silently rastering a blank, and refuses with a reason only when no snapshot is available. Without the flag+override, auto captures a blank overlay area silently. See examples/webview-plugin WebViewEditorPane for the pattern and the screenshot skill for the dispatch contract.

Canvas2D bridge gotchas (when the editor is on the @pulp/react native path, NOT WebView)

When porting a WebView editor to the native bridge (translating browser <canvas> + Canvas2D code to pulp's canvas* bridge globals via a JS shim like Spectr's canvas2d-shim.ts), several browser idioms silently break. Authoritative reference is the import-design skill's "Canvas2D Bridge Gotchas" section — see that for the full rule set with example code. Top-of-mind summary:

1. ctx.arc() does not add to a path — bridge canvasArc strokes immediately. Synthesize as ~32 canvasLineTo segments so ctx.fill() honors the active gradient. Same applies to arcTo, ellipse, roundRect.
2. Gradient stops must be spread as variadic (color, pos) pairs — passing JSON makes the bridge parse zero stops → silent fall-through to default white fill. canvasSetLinearGradient(id, x0, y0, x1, y1, c1, p1, c2, p2, ...).
3. Radial gradient bridge takes single outer circle (cx, cy, R), not the spec's two-circle form. Map (x0,y0,r0,x1,y1,r1) → (x1, y1, r1) (visually identical when r0=0, same center).
4. Per-canvas save_layer isolation — since pulp v0.74.1, ctx.clearRect() on one canvas no longer erases pixels another sibling just painted. Pin SDK >= 0.74.1 for any multi-canvas editor.
5. SDK version floor for canvas2d parity is v0.74.1 (sibling isolation) on top of v0.72.4 (gradient bridge) + v0.72.5 (gradient fills). Anything older has visible gaps.
6. Always pixel-sample after rendering — uniform-fallback bugs (every gradient stop resolved to default white) look superficially "filled" at thumbnail scale but are structurally broken.