Unify and improve download interactions

With help from Palid.

This does two major things:
1. Makes the tile part of a file body clickable to trigger a download.
2. Refactors a lot of the recyclable code out of the DownloadActionButton in favour of a utility. It's not a perfect refactoring, but it sets the stage for future work in the area (if needed).

The refactoring still has a heavy reliance on being supplied an iframe, but simplifies the DownloadActionButton and a hair of the MFileBody download code. In future, we'd probably want to make the iframe completely managed by the FileDownloader rather than have it only sometimes manage a hidden iframe.

src/utils/FileDownloader.ts

@@ -0,0 +1,104 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+export type getIframeFn = () => HTMLIFrameElement;
+
+export const DEFAULT_STYLES = {
+    imgSrc: "",
+    imgStyle: null, // css props
+    style: "",
+    textContent: "",
+}
+
+type DownloadOptions = {
+    blob: Blob,
+    name: string,
+    autoDownload?: boolean,
+    opts?: typeof DEFAULT_STYLES
+}
+
+// set up the iframe as a singleton so we don't have to figure out destruction of it down the line.
+let managedIframe: HTMLIFrameElement;
+let onLoadPromise: Promise<void>;
+function getManagedIframe(): { iframe: HTMLIFrameElement, onLoadPromise: Promise<void> } {
+    if (managedIframe) return { iframe: managedIframe, onLoadPromise };
+
+    managedIframe = document.createElement("iframe");
+
+    // Need to append the iframe in order for the browser to load it.
+    document.body.appendChild(managedIframe);
+
+    // Dev note: the reassignment warnings are entirely incorrect here.
+
+    // @ts-ignore
+    // noinspection JSConstantReassignment
+    managedIframe.style = { display: "none" };
+    // @ts-ignore
+    // noinspection JSConstantReassignment
+    managedIframe.sandbox = "allow-scripts allow-downloads allow-downloads-without-user-activation";
+
+    onLoadPromise = new Promise(resolve => {
+        managedIframe.onload = () => {
+            resolve();
+        };
+        managedIframe.src = "usercontent/"; // XXX: Should come from the skin
+    });
+
+    return { iframe: managedIframe, onLoadPromise };
+}
+
+// TODO: If we decide to keep the download link behaviour, we should bring the style management into here.
+
+/**
+ * Helper to handle safe file downloads. This operates off an iframe for reasons described
+ * by the blob helpers. By default, this will use a hidden iframe to manage the download
+ * through a user content wrapper, but can be given an iframe reference if the caller needs
+ * additional control over the styling/position of the iframe itself.
+ */
+export class FileDownloader {
+    private onLoadPromise: Promise<void>;
+
+    /**
+     * Creates a new file downloader
+     * @param iframeFn Function to get a pre-configured iframe. Set to null to have the downloader
+     * use a generic, hidden, iframe.
+     */
+    constructor(private iframeFn: getIframeFn = null) {
+    }
+
+    private get iframe(): HTMLIFrameElement {
+        const iframe = this.iframeFn?.();
+        if (!iframe) {
+            const managed = getManagedIframe();
+            this.onLoadPromise = managed.onLoadPromise;
+            return managed.iframe;
+        }
+        this.onLoadPromise = null;
+        return iframe;
+    }
+
+    public async download({blob, name, autoDownload = true, opts = DEFAULT_STYLES}: DownloadOptions) {
+        const iframe = this.iframe; // get the iframe first just in case we need to await onload
+        console.log("@@", {blob, name, autoDownload, opts, iframe, m: iframe === managedIframe, p: this.onLoadPromise});
+        if (this.onLoadPromise) await this.onLoadPromise;
+        iframe.contentWindow.postMessage({
+            ...opts,
+            blob: blob,
+            download: name,
+            auto: autoDownload,
+        }, '*');
+    }
+}