diff --git a/TESTING.md b/TESTING.md
index de08297..8069def 100644
--- a/TESTING.md
+++ b/TESTING.md
@@ -109,6 +109,13 @@ the baseline updater. Raise floors with `npm run --prefix tools/vendor
 coverage:update`; it reruns the suites and cannot lower an existing floor.
 Review baseline changes like code changes.
 
+`npm run --prefix tools/vendor coverage:baseline-check` runs in well under a
+second without a browser and fails, naming the files, when a measured
+`web/js` file has no baseline entry or the baseline lists a file that no longer
+exists. It uses the gate's own `discoverSources`, and `npm test` runs the same
+check, so run it before pushing a new or deleted frontend file instead of
+waiting for the `frontend coverage` job.
+
 The `viewerjs` check runs on every PR, including backend-only changes. It is an
 aggregate job that passes only when the `frontend coverage (chromium and touch)`
 job and both `page workflows (firefox)` and `page workflows (webkit)` jobs
diff --git a/tests/browser/library-concept.spec.mjs b/tests/browser/library-concept.spec.mjs
index 1589ad2..afc73ae 100644
--- a/tests/browser/library-concept.spec.mjs
+++ b/tests/browser/library-concept.spec.mjs
@@ -158,6 +158,10 @@ async function filteredOf(page,apply,undo) {
     await apply();
     await expect.poll(()=>captionOf(page)).not.toBe(before);
     await expect.poll(()=>page.locator(CARDS).count()).toBeLessThan(CONCEPT_PROMPTS);
+    // Production empties the shelf when the filter applies, writes the answer's caption at once and places its cards
+    // in the next animation frame (one paint for a viewer), so the empty shelf already counts as "fewer": read the
+    // titles only after that frame, as the Images and Inspector comparisons do.
+    await settle(page);
     const result={caption:await captionOf(page),titles:await titlesOf(page)};
     await undo();
     await expect.poll(()=>captionOf(page)).toBe(before);
diff --git a/tests/browser/workspace-images.spec.mjs b/tests/browser/workspace-images.spec.mjs
index 05ad6bc..0e799ed 100644
--- a/tests/browser/workspace-images.spec.mjs
+++ b/tests/browser/workspace-images.spec.mjs
@@ -1487,6 +1487,30 @@ test('a star press leaves the continuation status, count and caption date untouc
     expect(await page.evaluate(() => window.__statusMutations)).toEqual([]);
 });
 
+// Opened before the source and folder discovery answered (a slow server), Refine lists them once it does.
+test('Refine opened before discovery settles lists the discovered sources and folders', async ({ page }) => {
+    let release, started;
+    const held = new Promise(resolve => { release = resolve; }), hit = new Promise(resolve => { started = resolve; });
+    await page.route('**/prompt_manager/gallery/subfolders?*', async route => { started(); await held; await route.continue(); }, { times: 1 });
+    await page.goto(workspace);
+    await expect(cards(page)).toHaveCount(2);
+    await hit;
+    await filtersButton(page).click();
+    const source = refineDialog(page).getByLabel('Source', { exact: true });
+    const folder = refineDialog(page).getByLabel('Folder', { exact: true });
+    const values = select => select.locator('option').evaluateAll(options => options.map(option => option.value));
+    expect(await values(source)).toEqual(['all']);
+    expect(await values(folder)).toEqual(['']);
+    release();
+    await expect.poll(() => values(source)).toEqual(['all', '0', '1']);
+    expect(await values(folder)).toEqual(['', 'shared']);
+    await expect(source).toHaveValue('all');
+    await source.selectOption('1');
+    await refineDialog(page).getByRole('button', { name: 'Apply filters', exact: true }).click();
+    await expect(cards(page)).toHaveCount(1);
+    await expect(cards(page)).toHaveAttribute('data-image-key', '1:shared/same.png');
+});
+
 // #249 final review m1: as in the old field row, a Source offers only its own folders.
 test('Refine offers only the chosen source folders', async ({ page, request }) => {
     await expect(await request.post('/__test__/add-nested-image')).toBeOK();
diff --git a/tests/js/workspace-toolbar.test.js b/tests/js/workspace-toolbar.test.js
index bf01384..789569d 100644
--- a/tests/js/workspace-toolbar.test.js
+++ b/tests/js/workspace-toolbar.test.js
@@ -342,6 +342,56 @@ test('refine dialog round-trips values and clear resets them', async () => {
     }
 });
 
+test('Refine lists options discovered after it opened and keeps the wanted values', async () => {
+    const { openRefineDialog, openImportDialog, closeWorkspaceDialog } = await dialogsModule;
+    const dom = fakeDOM();
+    const scope = (await coreModule).createScope();
+    try {
+        let bySource = {};
+        const optionsFor = source => [['', 'All output folders'], ...(bySource[source] ?? []).map(path => [path, path])];
+        const fields = [{ key: 'source', options: [['all', 'All sources']] }, { key: 'folder', options: optionsFor('all'), optionsFor },
+            REFINE_FIELDS[2]];
+        const applied = [];
+        const refine = openRefineDialog({ scope, fields, values: { source: '1', folder: 'temp-only', media: 'video' },
+            onApply: value => applied.push(value), onClear() {} });
+        const [source, folder, media] = byId(dom, 'modal-body').children.filter(child => child.className === 'field')
+            .map(field => field.children[1]);
+        // Opened before discovery: only the cleared values.
+        assert.deepEqual(optionPairs(source), [['all', 'All sources']]);
+        assert.equal(source.value, 'all');
+        assert.equal(folder.value, '');
+        bySource = { all: ['portraits', 'temp-only'], 0: ['portraits'], 1: ['temp-only'] };
+        refine.setOptions('source', [['all', 'All sources'], ['0', 'output'], ['1', 'archive']]);
+        // The values it opened with, now offered, and the chosen Source's folders.
+        assert.deepEqual(optionPairs(source).map(([value]) => value), ['all', '0', '1']);
+        assert.equal(source.value, '1');
+        assert.deepEqual(optionPairs(folder), [['', 'All output folders'], ['temp-only', 'temp-only']]);
+        assert.equal(folder.value, 'temp-only');
+        // A choice made since wins over the opening value; tags and unknown keys are ignored.
+        source.value = '0';
+        source.dispatchEvent(new Event('change'));
+        refine.setOptions('source', [['all', 'All sources'], ['0', 'output'], ['1', 'archive']]);
+        refine.setOptions('tags', [['x', 'x']]);
+        assert.equal(source.value, '0');
+        assert.deepEqual(optionPairs(folder).map(([value]) => value), ['', 'portraits']);
+        assert.equal(media.value, 'video');
+        button(byId(dom, 'modal-body'), 'Apply filters').click();
+        assert.deepEqual(applied, [{ source: '0', folder: '', media: 'video' }]);
+        // Closed: a late discovery leaves the shell alone.
+        refine.setOptions('source', [['all', 'All sources']]);
+        assert.equal(source.options.length, 3);
+        // Replaced by another dialog: the same.
+        const again = openRefineDialog({ scope, fields, values: {}, onApply() {}, onClear() {} });
+        openImportDialog({ scope, onInspect() {}, onRescan() {} });
+        again.setOptions('source', [['all', 'All sources'], ['9', 'late']]);
+        assert.equal(byId(dom, 'modal-title').textContent, 'Import & inspect');
+    } finally {
+        closeWorkspaceDialog();
+        scope.dispose();
+        dom.restore();
+    }
+});
+
 test('the Folder list follows the chosen Source', async () => {
     const { openRefineDialog, closeWorkspaceDialog } = await dialogsModule;
     const dom = fakeDOM();
diff --git a/tests/vendor/coverage-baseline-check.test.mjs b/tests/vendor/coverage-baseline-check.test.mjs
new file mode 100644
index 0000000..187d174
--- /dev/null
+++ b/tests/vendor/coverage-baseline-check.test.mjs
@@ -0,0 +1,33 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { readFileSync } from 'node:fs';
+import { baselineGaps, measuredSources, readBaseline } from '../../tools/vendor/coverage-baseline-check.mjs';
+
+const floor = { lines: 80, branches: 80, functions: 80, statements: 80 };
+
+test('baseline gaps name measured files without a baseline entry', () => {
+    const gaps = baselineGaps(['a.js', 'nested/new.mjs'], { 'a.js': floor });
+    assert.deepEqual(gaps.missing, ['nested/new.mjs']);
+    assert.deepEqual(gaps.stale, []);
+});
+
+test('baseline gaps name baseline entries whose file no longer exists', () => {
+    const gaps = baselineGaps(['a.js'], { 'a.js': floor, 'gone.js': floor });
+    assert.deepEqual(gaps.missing, []);
+    assert.deepEqual(gaps.stale, ['gone.js']);
+});
+
+test('baseline gaps are empty when files and baseline agree', () => {
+    assert.deepEqual(baselineGaps(['a.js'], { 'a.js': floor }), { missing: [], stale: [] });
+});
+
+test('the committed baseline covers exactly the measured frontend sources', () => {
+    const gaps = baselineGaps(measuredSources(), readBaseline());
+    assert.deepEqual(gaps.missing, [], `Missing baseline (run coverage:update): ${gaps.missing.join(', ')}`);
+    assert.deepEqual(gaps.stale, [], `Stale baseline entries (file removed): ${gaps.stale.join(', ')}`);
+});
+
+test('the real gate and this check select files through the same function', () => {
+    const gate = readFileSync(new URL('../../tools/vendor/frontend-coverage.mjs', import.meta.url), 'utf8');
+    assert.match(gate, /discoverSources\(sourceRoot\)/);
+});
diff --git a/tools/vendor/coverage-baseline-check.mjs b/tools/vendor/coverage-baseline-check.mjs
new file mode 100644
index 0000000..795ccaf
--- /dev/null
+++ b/tools/vendor/coverage-baseline-check.mjs
@@ -0,0 +1,38 @@
+// Fast, browser-free check that frontend-coverage-baseline.json lists exactly the
+// files the coverage gate measures. CI's `frontend coverage` job reports a missing
+// entry only after ~20 minutes; this reports it in milliseconds.
+import { readFileSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+import { fileURLToPath, pathToFileURL } from 'node:url';
+import { repositoryRoot } from './vendor-viewerjs.mjs';
+import { discoverSources } from './frontend-coverage.mjs';
+
+const baselinePath = join(dirname(fileURLToPath(import.meta.url)), 'frontend-coverage-baseline.json');
+
+export function measuredSources() {
+    return discoverSources(join(repositoryRoot, 'web/js'));
+}
+
+export function readBaseline() {
+    return JSON.parse(readFileSync(baselinePath, 'utf8'));
+}
+
+export function baselineGaps(files, baseline) {
+    const measured = new Set(files);
+    return {
+        missing: files.filter(file => !baseline[file]),
+        stale: Object.keys(baseline).filter(file => !measured.has(file)).sort(),
+    };
+}
+
+function main() {
+    const { missing, stale } = baselineGaps(measuredSources(), readBaseline());
+    if (!missing.length && !stale.length) {
+        console.log('Coverage baseline matches the measured frontend files.');
+        return;
+    }
+    if (missing.length) console.error(`Missing baseline for:\n  ${missing.join('\n  ')}\nRun: npm run --prefix tools/vendor coverage:update`);
+    if (stale.length) console.error(`Baseline lists files that no longer exist:\n  ${stale.join('\n  ')}\nRemove them from tools/vendor/frontend-coverage-baseline.json`);
+    process.exitCode = 1;
+}
+if (process.argv[1] && import.meta.url === pathToFileURL(process.argv[1]).href) main();
diff --git a/tools/vendor/package.json b/tools/vendor/package.json
index 0fda145..c64b903 100644
--- a/tools/vendor/package.json
+++ b/tools/vendor/package.json
@@ -15,6 +15,7 @@
     "test:comfyui": "playwright test --config comfyui.config.mjs",
     "test:executions": "playwright test --config playwright.execution.config.mjs",
     "test:coverage": "node frontend-coverage.mjs",
+    "coverage:baseline-check": "node coverage-baseline-check.mjs",
     "coverage:update": "node frontend-coverage.mjs --update-baseline",
     "test:compat": "playwright test --config pack-compat.config.mjs"
   },
diff --git a/web/js/workspace/images.mjs b/web/js/workspace/images.mjs
index 6ef6e6c..214358f 100644
--- a/web/js/workspace/images.mjs
+++ b/web/js/workspace/images.mjs
@@ -134,6 +134,8 @@ export function mountImages(main, api, parentScope, {embedded = false, fetchPage
     let state, geometry = { positions: [], columns: 1, height: 0, maxHeight: 0 }, galleryWidth = 0, optionsRequest, categoryRequest, recovering = false, rescanning = false;
     // Discovered sources (each with its own folders) and every folder, for "Refine this view" and the scope chip.
     let roots = [], folders = [];
+    // The open "Refine this view", so a discovery that settles after it opened still lists its sources and folders.
+    let refine = null;
     const rendered = new Map();
     const itemIndexes = new Map();
     const filters = { root: 'all', q: '', category: '', tags: [], subfolder: '', media: 'all', sort: 'newest', ...visit?.filters, ...saved,
@@ -646,8 +648,8 @@ export function mountImages(main, api, parentScope, {embedded = false, fetchPage
     /** "Refine this view": Source, Folder, Media and Tags, the filters the old field row held (D14). */
     function openFilters(trigger) {
         if (recovering) return;
-        openRefineDialog({scope, trigger, fields:[
-            {key:'source', options:[['all', 'All sources'], ...roots.map(root => [String(root.index), root.label])]},
+        refine = openRefineDialog({scope, trigger, fields:[
+            {key:'source', options:sourceOptions()},
             {key:'folder', options:folderOptions('all'), optionsFor:folderOptions},
             {key:'media', options:MEDIA_OPTIONS},
             {key:'tags'},
@@ -657,6 +659,9 @@ export function mountImages(main, api, parentScope, {embedded = false, fetchPage
             applyRefinement(cleared);
         }});
     }
+    function sourceOptions() {
+        return [['all', 'All sources'], ...roots.map(root => [String(root.index), root.label])];
+    }
     /** The chosen Source's folders, as the old field row reloaded them; every folder under All sources. */
     function folderOptions(source) {
         const root = source === 'all' ? null : roots.find(item => String(item.index) === source);
@@ -753,6 +758,7 @@ export function mountImages(main, api, parentScope, {embedded = false, fetchPage
             roots = data.roots.filter(root => Number.isSafeInteger(root?.index) && typeof root.label === 'string')
                 .map(root => ({index:root.index, label:root.label, subfolders:paths(root.subfolders)}));
             folders = paths(data.subfolders);
+            refine?.setOptions('source', sourceOptions());
             updateScope();
             sourcesRetry = null;
             return true;
diff --git a/web/js/workspace/workspace-dialogs.mjs b/web/js/workspace/workspace-dialogs.mjs
index c4f9d14..6872a68 100644
--- a/web/js/workspace/workspace-dialogs.mjs
+++ b/web/js/workspace/workspace-dialogs.mjs
@@ -155,6 +155,9 @@ function choose(select, value) {
  * Tags; Folder may add `optionsFor(source)`, the options of one Source, so its list follows the chosen Source and a
  * folder that Source lacks returns to the first option. `values` holds the current value per key. Apply reports the edited values; Clear resets every field and
  * reports the cleared values plus `category: ''`, so the page clears its toolbar category too. Both close first.
+ * Returns `{setOptions(key, options)}` for options discovered after the dialog opened (a slow folder discovery): the
+ * select is refilled and shows the value the field opened with, or the one chosen since, once it is offered; a Folder
+ * that follows its Source is refilled from `optionsFor` again. It does nothing once this dialog closed or was replaced.
  */
 export function openRefineDialog({ scope, fields, values = {}, onApply, onClear, trigger = null }) {
     requireScope(scope);
@@ -175,16 +178,21 @@ export function openRefineDialog({ scope, fields, values = {}, onApply, onClear,
         choose(select, values[key]);
         return [{ key, control: select, cleared: item.options[0][0], optionsFor: item.optionsFor }];
     });
+    // Each select's wanted value: the one it opened with until the person picks another (see setOptions).
+    const wanted = new Map(controls.map(({ key }) => [key, values[key]]));
+    for (const { key, control } of controls) {
+        if (key !== 'tags') control.addEventListener('change', () => wanted.set(key, control.value));
+    }
     const source = controls.find(item => item.key === 'source');
     const folder = controls.find(item => item.key === 'folder' && typeof item.optionsFor === 'function');
-    const followSource = () => {
-        const chosen = folder.control.value;
+    const followSource = (chosen = folder.control.value) => {
         fillSelect(folder.control, 'folder', folder.optionsFor(source.control.value));
         choose(folder.control, chosen);
     };
     if (source && folder) {
-        followSource();
-        source.control.addEventListener('change', followSource);
+        followSource(values.folder);
+        // A Source that lacks the chosen folder clears it, and that cleared folder is the wanted one from then on.
+        source.control.addEventListener('change', () => { followSource(); wanted.set('folder', folder.control.value); });
     }
     const read = () => Object.fromEntries(controls.map(({ key, control }) =>
         [key, key === 'tags' ? control.value.trim() : control.value]));
@@ -210,6 +218,15 @@ export function openRefineDialog({ scope, fields, values = {}, onApply, onClear,
         ...controls.map(({ key, control }) => field(FIELD_LABELS[key], control, `filter-${key}`)),
         actions,
     ], { scope, trigger });
+    const opened = session;
+    function setOptions(key, options) {
+        const item = controls.find(candidate => candidate.key === key);
+        if (session !== opened || !item || key === 'tags') return;
+        fillSelect(item.control, key, options);
+        choose(item.control, wanted.get(key));
+        if (source && folder && (item === source || item === folder)) followSource(wanted.get('folder'));
+    }
+    return { setOptions };
 }
 
 /** One operation row; a press on its busy (aria-disabled) action is ignored and keeps the dialog open. */