VaCuus

HTML/CSS user interface for Unreal Engine, rendered off the game thread.

You author screens as .rml/.rcss documents — the web languages, not a UMG graph — and the plugin runs them on its own UI thread: layout, styling and draw-command recording all happen away from the game thread, which only enqueues input and reads back a snapshot. Optional JavaScript (QuickJS) and a TypeScript/Preact workflow sit on top. Rendering goes through the engine's RHI into a persistent render target that Slate composites, so a document can be a full-screen HUD or a panel on a quad in the world.

Status: released, 1.0.3. Engines: UE 5.6, 5.7 and 5.8. All three are built and tested from this one source tree — editor and packaged-game targets, plus the 260-test automation suite that ships with the plugin and that you can run yourself (below). Every shipping platform is supported: Windows, macOS, Linux, Android and iOS; consoles on request. Packages are published on GitHub Releases, one archive per engine (VaCuus-<version>-UE5.8.zip and its 5.6 and 5.7 siblings), each stamped with the engine that built it — take the one that matches yours (docs/buyer/setup.md says how to check). The API is stable — breaking changes are reserved for major versions and carry release notes.

Using Claude Code, Codex, Cursor or Copilot? One line in your project's own CLAUDE.md / AGENTS.md is enough to point it at the right pages:

UI is VaCuus (HTML/CSS off the game thread). Before touching anything under
Content/DevUI, read Plugins/VaCuus/docs/buyer/ai-guide.md and
Plugins/VaCuus/docs/buyer/rcss-matrix.md.

From Fab: the launcher installs the plugin into the engine, not into your project — Epic builds the binaries for code plugins, so nothing compiles on your machine and a Blueprint-only project works. Enabling it is the whole install. From source (a release archive or this repository): your project must be a C++ project — a Blueprint-only project has no toolchain to compile it with (converting takes two minutes: Tools → New C++ Class → None). Drop the folder into <YourProject>/Plugins/VaCuus, so that VaCuus.uplugin lands at <YourProject>/Plugins/VaCuus/VaCuus.uplugin, and the first editor launch compiles the modules once. Either way, enable it in Edit → Plugins or in your .uproject:

"Plugins": [
    {
        "Name": "VaCuus",
        "Enabled": true
    }
]

The package root is the plugin folder — there is no extra directory level to strip. setup.md §0 has the full version of this, including the two ways a shipped copy differs from the repository.

Verify it works: the console knows vacuus.M2Demo — run it and an interactive demo document appears. Automation RunTests VaCuus runs the suite; the test source and its fixtures ship on purpose.

A symlink instead of a copy works at runtime but breaks the build: Unreal Build Accelerator's file detours see the symlinked path and the real path as two different files and abort renaming an object file (cross-process rename-while-open). To keep the plugin outside the project, use AdditionalPluginDirectories in your .uproject instead — and point it at a parent of the plugin directory, never at the plugin directory itself.

VaCuus.uplugin              Plugin descriptor
Content/DevUI/              The RML/RCSS documents the plugin ships, demos and test fixtures
Resources/Icon128.png       Icon shown in the editor plugin browser
Shaders/                    The UI shaders, including the scene-colour backdrop path
Source/ThirdParty/RmlUi/    Vendored RmlUi 6.x — bit-exact upstream plus recorded patches
Source/ThirdParty/quickjs-ng/  Vendored QuickJS-ng, the JavaScript runtime
Source/VaCuusRml/           The vendored library, built as a module (Runtime, Default)
Source/VaCuus/              Runtime core: the UI thread, VFS, subsystem, views (Runtime, Default)
Source/VaCuusRender/        Record -> RHI replay -> Slate composite, UMG widget (Runtime, PostConfigInit)
Source/VaCuusJs/            The JavaScript host and the DOM facade (Runtime, Default)
Source/VaCuusEditor/        Live reload and the bundle factory (Editor, PostEngineInit)
Web/                        Optional TypeScript/Preact workflow, source-only (npm-install it yourself)
docs/buyer/                 The five pages above, plus localization.md

There is no Binaries/ directory in the package and that is Fab's rule for sellers of UE-facing code: what we upload is source. The binaries a Fab install carries are Epic's build of that source, and they land in the engine — a source install compiles them on your machine instead.

Every RmlUi call happens on the UI thread; the game thread only enqueues commands and reads a published interactive-region snapshot. Runtime code logs under the LogVaCuus category declared in Source/VaCuus/Public/VaCuusDefines.h — that is the category to raise to Verbose when something is quiet.

Both are vendored in-tree with their licenses beside them, and both are MIT:

• RmlUi 6.x — Source/ThirdParty/RmlUi/LICENSE.txt. The document, layout and styling engine. Patches applied to it are recorded rather than silent; see VENDORED_TAG.txt.
• QuickJS-ng — Source/ThirdParty/quickjs-ng/LICENSE. The JavaScript runtime, with its symbols deliberately kept out of the module's export table.

The plugin's own source is licensed under the Business Source License 1.1 (LICENSE.md): free to read, modify and use outside production, free in production for noncommercial purposes, and each version converts to MIT four years after release. Commercial production use needs a purchased license — on Fab (under the Fab EULA) or directly from the author; COMMERCIAL.md is the plain-language version. A copy bought on a store is governed by that store's terms. Contributions require the CLA (CLA.md).

Issues and questions: https://github.com/ufna/VaCuus/issues.

Working on the plugin itself rather than with it? The host-project layout, the two-working-trees hazard and the arrangements that do and do not remove the duplication are in docs/dev/host-project.md, which is repo-only.