files: readable text previews in dark themes (#697)

* files: readable text previews in dark themes

A same-origin text document inherits the page's color-scheme, so dark
themes rendered the browser's default light text on the preview's
forced white background. Force a light color-scheme on the preview
iframe so text is always black on white.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* files: text previews follow the theme

Instead of pinning the preview iframe to a light scheme, follow the
theme: dark, night, and amber render the document light-on-dark against
the theme's panel color; light stays black-on-white.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

app/src/components/FilePreview.js

@@ -100,8 +100,8 @@ function getEpubViewerURL(previewFile) {
  * A component rather than part of `getIframePreviewModal` because it reads the theme: the
  * modal builders are plain functions called from an event handler, where hooks cannot run.
  */
-function IframePreview({url, gatePdf}) {
-    const {mediaFilterEnabled} = React.useContext(ThemeContext);
+export function IframePreview({url, gatePdf}) {
+    const {isDark, mediaFilterEnabled} = React.useContext(ThemeContext);
 
     /*
      * `gatePdf` only for PDFs.  Text and the EPUB viewer are documents of our own, which the
@@ -119,8 +119,12 @@ function IframePreview({url, gatePdf}) {
             <iframe title='textModal' src={url}
                     style={{
                         height: '100%', width: '100%', border: 'none',
-                        // Use white to avoid iframe displaying with dark-theme.
-                        backgroundColor: '#ffffff',
+                        // A same-origin document has no styles of its own; it renders with the
+                        // browser's default stylesheet for whatever color-scheme this element
+                        // declares.  Pin the scheme to the theme so text is light-on-dark under
+                        // the dark themes and black-on-white under light, never white-on-white.
+                        backgroundColor: isDark ? 'var(--panel)' : '#ffffff',
+                        colorScheme: isDark ? 'dark' : 'light',
                     }}/>
         </div>
     </MediaGate>

app/src/components/FilePreview.test.js

@@ -100,3 +100,45 @@ describe('preview gate remounting (source contract)', () => {
         expect(rebuild.length).toBeGreaterThan(0);
     });
 });
+
+describe('IframePreview color scheme', () => {
+    /*
+     * The previewed file is a bare document with no styles of its own, so it renders with the
+     * browser's default stylesheet for whatever color-scheme the iframe element declares.  If
+     * the scheme does not follow the theme, one combination is always unreadable: a dark page
+     * scheme with a white iframe renders the browser's light text on white.
+     */
+    const {render} = require('@testing-library/react');
+    const {ThemeContext} = require('../contexts/contexts');
+    const {IframePreview} = require('./FilePreview');
+
+    const renderWithTheme = (isDark) => render(
+        <ThemeContext.Provider value={{isDark, mediaFilterEnabled: false}}>
+            <IframePreview url='/media/config/wrolpi.yaml'/>
+        </ThemeContext.Provider>
+    );
+
+    test('dark themes render the document dark with a dark background', () => {
+        const {container} = renderWithTheme(true);
+        const iframe = container.querySelector('iframe');
+        expect(iframe.style.colorScheme).toEqual('dark');
+    });
+
+    test('the dark background is the theme panel color', () => {
+        /*
+         * Read off the source because jsdom's CSSOM drops var() declarations entirely, so a
+         * rendered iframe in jsdom cannot show the background it gets in a real browser.
+         */
+        const fs = require('fs');
+        const path = require('path');
+        const source = fs.readFileSync(path.join(__dirname, 'FilePreview.js'), 'utf8');
+        expect(source).toContain(`backgroundColor: isDark ? 'var(--panel)' : '#ffffff'`);
+    });
+
+    test('the light theme renders the document black-on-white', () => {
+        const {container} = renderWithTheme(false);
+        const iframe = container.querySelector('iframe');
+        expect(iframe.style.colorScheme).toEqual('light');
+        expect(iframe.style.backgroundColor).toEqual('rgb(255, 255, 255)');
+    });
+});