diff --git a/res/decoder-ring/datatypes.js b/res/decoder-ring/datatypes.js
new file mode 100644
index 000000000..93a779e07
--- /dev/null
+++ b/res/decoder-ring/datatypes.js
@@ -0,0 +1,107 @@
+/*
+ * Quick-n-dirty algebraic datatypes.
+ *
+ * These let us handle the possibility of failure without having to constantly write code to check for it.
+ * We can apply all of the transformations we need as if the data is present using `map`.
+ * If there's a None, or a FetchError, or a Pending, those are left untouched.
+ *
+ * I've used perhaps an odd bit of terminology from scalaz in `fold`.  This is basically a `switch` statement:
+ * You pass it a set of functions to handle the various different states of the datatype, and if it finds the
+ * function it'll call it on its value.
+ *
+ * It's handy to have this in functional style when dealing with React as we can dispatch different ways of rendering
+ * really simply:
+ * ```
+ * bundleFetchStatus.fold({
+ *     some: (fetchStatus) => <ProgressBar fetchsStatus={fetchStatus} />,
+ * }),
+ * ```
+ */
+
+
+class Optional {
+    static from(value) {
+        return value && Some.of(value) || None;
+    }
+    map(f) {
+        return this;
+    }
+    flatMap(f) {
+        return this;
+    }
+    fold({ none }) {
+        return none && none();
+    }
+}
+class Some extends Optional {
+    constructor(value) {
+        super();
+        this.value = value;
+    }
+    map(f) {
+        return Some.of(f(this.value));
+    }
+    flatMap(f) {
+        return f(this.value);
+    }
+    fold({ some }) {
+        return some && some(this.value);
+    }
+    static of(value) {
+        return new Some(value);
+    }
+}
+const None = new Optional();
+
+class FetchStatus {
+    constructor(opt = {}) {
+        this.opt = { at: Date.now(), ...opt };
+    }
+    map(f) {
+        return this;
+    }
+    flatMap(f) {
+        return this;
+    }
+}
+class Success extends FetchStatus {
+    static of(value) {
+        return new Success(value);
+    }
+    constructor(value, opt) {
+        super(opt);
+        this.value = value;
+    }
+    map(f) {
+        return new Success(f(this.value), this.opt);
+    }
+    flatMap(f) {
+        return f(this.value, this.opt);
+    }
+    fold({ success }) {
+        return success instanceof Function ? success(this.value, this.opt) : undefined;
+    }
+}
+class Pending extends FetchStatus {
+    static of(opt) {
+        return new Pending(opt);
+    }
+    constructor(opt) {
+        super(opt);
+    }
+    fold({ pending }) {
+        return pending instanceof Function ? pending(this.opt) : undefined;
+    }
+}
+class FetchError extends FetchStatus {
+    static of(reason, opt) {
+        return new FetchError(reason, opt);
+    }
+    constructor(reason, opt) {
+        super(opt);
+        this.reason = reason;
+    }
+    fold({ error }) {
+        return error instanceof Function ? error(this.reason, this.opt) : undefined;
+    }
+}
diff --git a/res/decoder-ring/decoder.js b/res/decoder-ring/decoder.js
new file mode 100644
index 000000000..68c6bb066
--- /dev/null
+++ b/res/decoder-ring/decoder.js
@@ -0,0 +1,319 @@
+class StartupError extends Error {}
+
+/*
+ * We need to know the bundle path before we can fetch the sourcemap files.  In a production environment, we can guess
+ * it using this.
+ */
+async function getBundleName() {
+    const res = await fetch("../index.html");
+    if (!res.ok) {
+        throw new StartupError(`Couldn't fetch index.html to prefill bundle; ${res.status} ${res.statusText}`);
+    }
+    const index = await res.text();
+    return index.split("\n").map((line) =>
+        line.match(/<script src="bundles\/([^/]+)\/bundle.js"/),
+    )
+    .filter((result) => result)
+    .map((result) => result[1])[0];
+}
+
+function validateBundle(value) {
+    return value.match(/^[0-9a-f]{20}$/) ? Some.of(value) : None;
+}
+
+/* A custom fetcher that abandons immediately upon getting a response.
+ * The purpose of this is just to validate that the user entered a real bundle, and provide feedback.
+ */
+const bundleCache = new Map();
+function bundleSubject(bundle) {
+    if (!bundle.match(/^[0-9a-f]{20}$/)) throw new Error("Bad input");
+    if (bundleCache.has(bundle)) {
+        return bundleCache.get(bundle);
+    }
+    const fetcher = new rxjs.BehaviorSubject(Pending.of());
+    bundleCache.set(bundle, fetcher);
+
+    fetch(`/bundles/${bundle}/bundle.js.map`).then((res) => {
+        res.body.cancel(); /* Bail on the download immediately - it could be big! */
+        const status = res.ok;
+        if (status) {
+            fetcher.next(Success.of());
+        } else {
+            fetcher.next(FetchError.of(`Failed to fetch: ${res.status} ${res.statusText}`));
+        }
+    });
+
+    return fetcher;
+}
+
+/*
+ * Convert a ReadableStream of bytes into an Observable of a string
+ * The observable will emit a stream of Pending objects and will concatenate
+ * the number of bytes received with whatever pendingContext has been supplied.
+ * Finally, it will emit a Success containing the result.
+ * You'd use this on a Response.body.
+ */
+function observeReadableStream(readableStream, pendingContext = {}) {
+    let bytesReceived = 0;
+    let buffer = "";
+    const pendingSubject = new rxjs.BehaviorSubject(Pending.of({ ...pendingContext, bytesReceived }));
+    const throttledPending = pendingSubject.pipe(rxjs.operators.throttleTime(100));
+    const resultObservable = new rxjs.Subject();
+    const reader = readableStream.getReader();
+    const utf8Decoder = new TextDecoder("utf-8");
+    function readNextChunk() {
+        reader.read().then(({ done, value }) => {
+            if (done) {
+                pendingSubject.complete();
+                resultObservable.next(Success.of(buffer));
+                return;
+            }
+            bytesReceived += value.length;
+            pendingSubject.next(Pending.of({...pendingContext, bytesReceived }));
+            /* string concatenation is apparently the most performant way to do this */
+            buffer += utf8Decoder.decode(value);
+            readNextChunk();
+        });
+    }
+    readNextChunk();
+    return rxjs.concat(throttledPending, resultObservable);
+}
+
+/*
+ * A wrapper which converts the browser's `fetch()` mechanism into an Observable.  The Observable then provides us with
+ * a stream of datatype values: first, a sequence of Pending objects that keep us up to date with the download progress,
+ * finally followed by either a Success or Failure object.  React then just has to render each of these appropriately.
+ */
+const fetchCache = new Map();
+function fetchAsSubject(endpoint) {
+    if (fetchCache.has(endpoint)) {
+        // TODO: expiry/retry logic here?
+        return fetchCache.get(endpoint);
+    }
+    const fetcher = new rxjs.BehaviorSubject(Pending.of());
+    fetchCache.set(endpoint, fetcher);
+
+    fetch(endpoint).then((res) => {
+        if (!res.ok) {
+            fetcher.next(FetchError.of(`Failed to fetch endpoint ${endpoint}: ${res.status} ${res.statusText}`));
+            return;
+        }
+
+        const contentLength = res.headers.get("content-length");
+        const context = contentLength ? { length: parseInt(contentLength) } : {};
+
+        const streamer = observeReadableStream(res.body, context, endpoint);
+        streamer.subscribe((value) => {
+            fetcher.next(value);
+        });
+    });
+    return fetcher;
+}
+
+/* ===================== */
+/* ==== React stuff ==== */
+/* ===================== */
+/* Rather than importing an entire build infrastructure, for now we just use React without JSX */
+const e = React.createElement;
+
+/*
+ * Provides user feedback given a FetchStatus object.
+ */
+function ProgressBar({ fetchStatus }) {
+    return e('span', { className: "progress "},
+        fetchStatus.fold({
+            pending: ({ bytesReceived, length }) => {
+                if (!bytesReceived) {
+                    return e('span', { className: "spinner" }, "\u29b5");
+                }
+                const kB = Math.floor(10 * bytesReceived / 1024) / 10;
+                if (!length) {
+                    return e('span', null, `Fetching (${kB}kB)`);
+                }
+                const percent = Math.floor(100 * bytesReceived / length);
+                return e('span', null, `Fetching (${kB}kB) ${percent}%`);
+            },
+            success: () => e('span', null, "\u2713"),
+            error: (reason) => {
+                return e('span', { className: 'error'}, `\u2717 ${reason}`);
+            },
+        },
+    ));
+}
+
+/*
+ * The main component.
+ */
+function BundlePicker() {
+    const [bundle, setBundle] = React.useState("");
+    const [file, setFile] = React.useState("");
+    const [line, setLine] = React.useState("1");
+    const [column, setColumn] = React.useState("");
+    const [result, setResult] = React.useState(None);
+    const [bundleFetchStatus, setBundleFetchStatus] = React.useState(None);
+    const [fileFetchStatus, setFileFetchStatus] = React.useState(None);
+
+    /* At startup, try to fill in the bundle name for the user */
+    React.useEffect(() => {
+        getBundleName().then((name) => {
+            if (bundle === "" && validateBundle(name) !== None) {
+                setBundle(name);
+            }
+        }, console.log.bind(console));
+    }, []);
+
+
+    /* ------------------------- */
+    /* Follow user state changes */
+    /* ------------------------- */
+    const onBundleChange = React.useCallback((event) => {
+        const value = event.target.value;
+        setBundle(value);
+    }, []);
+
+    const onFileChange = React.useCallback((event) => {
+        const value = event.target.value;
+        setFile(value);
+    }, []);
+
+    const onLineChange = React.useCallback((event) => {
+        const value = event.target.value;
+        setLine(value);
+    }, []);
+
+    const onColumnChange = React.useCallback((event) => {
+        const value = event.target.value;
+        setColumn(value);
+    }, []);
+
+
+    /* ------------------------------------------------ */
+    /* Plumb data-fetching observables through to React */
+    /* ------------------------------------------------ */
+
+    /* Whenever a valid bundle name is input, go see if it's a real bundle on the server */
+    React.useEffect(() =>
+        validateBundle(bundle).fold({
+            some: (value) => {
+                const subscription = bundleSubject(value)
+                    .pipe(rxjs.operators.map(Some.of))
+                    .subscribe(setBundleFetchStatus);
+                return () => subscription.unsubscribe();
+            },
+            none: () => setBundleFetchStatus(None),
+        }),
+    [bundle]);
+
+    /* Whenever a valid javascript file is input, see if it corresponds to a sourcemap file and initiate a fetch
+     * if so. */
+    React.useEffect(() => {
+        if (!file.match(/.\.js$/) || validateBundle(bundle) === None) {
+            setFileFetchStatus(None);
+            return;
+        }
+        const observable = fetchAsSubject(`/bundles/${bundle}/${file}.map`)
+            .pipe(
+                rxjs.operators.map((fetchStatus) => fetchStatus.flatMap(value => {
+                    try {
+                        return Success.of(JSON.parse(value));
+                    } catch (e) {
+                        return FetchError.of(e);
+                    }
+                })),
+                rxjs.operators.map(Some.of),
+            );
+        const subscription = observable.subscribe(setFileFetchStatus);
+        return () => subscription.unsubscribe();
+    }, [bundle, file]);
+
+    /*
+     * Whenever we have a valid fetched sourcemap, and a valid line, attempt to find the original position from the
+     * sourcemap.
+     */
+    React.useEffect(() => {
+        // `fold` dispatches on the datatype, like a switch statement
+        fileFetchStatus.fold({
+            some: (fetchStatus) =>
+                // `fold` just returns null for all of the cases that aren't `Success` objects here
+                fetchStatus.fold({
+                    success: (value) => {
+                        if (!line) return setResult(None);
+                        const pLine = parseInt(line);
+                        const pCol = parseInt(column);
+                        sourceMap.SourceMapConsumer.with(value, undefined, (consumer) =>
+                            consumer.originalPositionFor({ line: pLine, column: pCol }),
+                        ).then((result) => setResult(Some.of(JSON.stringify(result))));
+                    },
+                }),
+            none: () => setResult(None),
+        });
+    }, [fileFetchStatus, line, column]);
+
+
+    /* ------ */
+    /* Render */
+    /* ------ */
+    return e('div', {},
+        e('div', { className: 'inputs' },
+            e('div', { className: 'bundle' },
+                e('label', { htmlFor: 'bundle'}, 'Bundle'),
+                e('input', {
+                    name: 'bundle',
+                    required: true,
+                    pattern: "[0-9a-f]{20}",
+                    onChange: onBundleChange,
+                    value: bundle,
+                }),
+                bundleFetchStatus.fold({
+                    some: (fetchStatus) => e(ProgressBar, { fetchStatus }),
+                    none: () => null,
+                }),
+            ),
+            e('div', { className: 'file' },
+                e('label', { htmlFor: 'file' }, 'File'),
+                e('input', {
+                    name: 'file',
+                    required: true,
+                    pattern: ".+\\.js",
+                    onChange: onFileChange,
+                    value: file,
+                }),
+                fileFetchStatus.fold({
+                    some: (fetchStatus) => e(ProgressBar, { fetchStatus }),
+                    none: () => null,
+                }),
+            ),
+            e('div', { className: 'line' },
+                e('label', { htmlFor: 'line' }, 'Line'),
+                e('input', {
+                    name: 'line',
+                    required: true,
+                    pattern: "[0-9]+",
+                    onChange: onLineChange,
+                    value: line,
+                }),
+            ),
+            e('div', { className: 'column' },
+                e('label', { htmlFor: 'column' }, 'Column'),
+                e('input', {
+                    name: 'column',
+                    required: true,
+                    pattern: "[0-9]+",
+                    onChange: onColumnChange,
+                    value: column,
+                }),
+            ),
+        ),
+        e('div', null,
+            result.fold({
+                none: () => "Select a bundle, file and line",
+                some: (value) => e('pre', null, value),
+            }),
+        ),
+    );
+}
+
+/* Global stuff */
+window.Decoder = {
+    BundlePicker,
+};
diff --git a/res/decoder-ring/index.html b/res/decoder-ring/index.html
new file mode 100644
index 000000000..9984815f0
--- /dev/null
+++ b/res/decoder-ring/index.html
@@ -0,0 +1,79 @@
+<html>
+    <head>
+        <title>Rageshake decoder ring</title>
+        <script crossorigin src="https://unpkg.com/source-map@0.7.3/dist/source-map.js"></script>
+        <script>
+            sourceMap.SourceMapConsumer.initialize({
+                "lib/mappings.wasm": "https://unpkg.com/source-map@0.7.3/lib/mappings.wasm"
+            });
+        </script>
+        <script crossorigin src="https://unpkg.com/react@16/umd/react.production.min.js"></script>
+        <script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.production.min.js"></script>
+        <!--<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
+        <script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>-->
+        <script crossorigin src="https://unpkg.com/rxjs/bundles/rxjs.umd.min.js"></script>
+        <script src="datatypes.js"></script>
+        <script src="decoder.js"></script>
+
+        <style>
+            @keyframes spin {
+                from {transform:rotate(0deg);}
+                to {transform:rotate(359deg);}
+            }
+
+            body {
+                font-family: sans-serif
+            }
+
+            .spinner {
+                animation: spin 4s infinite linear;
+                display: inline-block;
+                text-align: center;
+                vertical-align: middle;
+                font-size: larger;
+            }
+
+            .progress {
+                padding-left: 0.5em;
+                padding-right: 0.5em;
+            }
+
+            .bundle input {
+                width: 24ex;
+            }
+
+            .valid::after {
+                content: "✓"
+            }
+
+            label {
+                width: 3em;
+                margin-right: 1em;
+                display: inline-block;
+            }
+
+            input:valid {
+                border: 1px solid green;
+            }
+
+            .inputs > div {
+                margin-bottom: 0.5em;
+            }
+        </style>
+    </head>
+    <body>
+        <header><h2>Decoder ring</h2></header>
+        <content id="main">Waiting for javascript to run...</content>
+        <script type="text/javascript">
+            document.addEventListener("DOMContentLoaded", () => {
+                try {
+                    ReactDOM.render(React.createElement(Decoder.BundlePicker), document.getElementById("main"))
+                } catch (e) {
+                    const n = document.createElement("div");
+                    n.innerText = `Error starting: ${e.message}`;
+                    document.getElementById("main").appendChild(n);
+                }
+            });
+        </script>
+    </body>
+</html>
diff --git a/scripts/copy-res.js b/scripts/copy-res.js
index d1fb49462..6153bb9b9 100755
--- a/scripts/copy-res.js
+++ b/scripts/copy-res.js
@@ -63,11 +63,11 @@ const COPY_LIST = [
     ["res/welcome/**", "webapp/welcome"],
     ["res/themes/**", "webapp/themes"],
     ["res/vector-icons/**", "webapp/vector-icons"],
+    ["res/decoder-ring/**", "webapp/decoder-ring"],
     ["node_modules/matrix-react-sdk/res/media/**", "webapp/media"],
     ["node_modules/olm/olm_legacy.js", "webapp", { directwatch: 1 }],
     ["./config.json", "webapp", { directwatch: 1 }],
     ["contribute.json", "webapp"],
-    ["node_modules/matrix-react-sdk/res/decoder-ring/**", "webapp/decoder-ring"],
 ];
 
 const parseArgs = require('minimist');