diff --git a/frontend/src/lib/diashow/queue.test.ts b/frontend/src/lib/diashow/queue.test.ts
index ca8a57d..f21be1a 100644
--- a/frontend/src/lib/diashow/queue.test.ts
+++ b/frontend/src/lib/diashow/queue.test.ts
@@ -2,16 +2,20 @@ import { describe, it, expect } from 'vitest';
 import { SlideQueue } from './queue';
 import type { FeedUpload } from '$lib/types';
 
-// Minimal FeedUpload factory — the queue only reads id, user_id, preview_url, thumbnail_url.
+// Minimal FeedUpload factory — the queue reads id, user_id, preview_url, thumbnail_url and
+// mime_type. Note the `as unknown as FeedUpload` cast below: it silences the missing fields,
+// so adding a field the queue reads does NOT fail typechecking here — it fails at runtime.
+// Keep this factory in step with what `merge` actually touches.
 const up = (
 	id: string,
-	opts: { user?: string; preview?: boolean; thumb?: boolean } = {}
+	opts: { user?: string; preview?: boolean; thumb?: boolean; video?: boolean } = {}
 ): FeedUpload =>
 	({
 		id,
 		user_id: opts.user ?? 'u1',
 		preview_url: opts.preview === false ? null : `/p/${id}`,
-		thumbnail_url: opts.thumb ? `/t/${id}` : null
+		thumbnail_url: opts.thumb ? `/t/${id}` : null,
+		mime_type: opts.video ? 'video/mp4' : 'image/jpeg'
 	}) as unknown as FeedUpload;
 
 const drain = (q: SlideQueue, n: number): string[] => {
@@ -32,6 +36,19 @@ describe('SlideQueue.merge', () => {
 		expect(q.stats().known).toBe(3);
 	});
 
+	it('never queues a video, even when it has a usable poster frame', () => {
+		const q = new SlideQueue();
+		// The projector shows stills only. Before this was keyed on the mime type it depended
+		// on whether ffmpeg happened to extract a poster: a video WITH a thumbnail was shown,
+		// one without was dropped — the same clip in or out by luck.
+		const added = q.merge([up('photo'), up('clip', { video: true, thumb: true })], {
+			live: false
+		});
+		expect(added).toBe(1);
+		expect(q.has('clip')).toBe(false);
+		expect(q.has('photo')).toBe(true);
+	});
+
 	it('skips uploads with no preview or thumbnail (still compressing)', () => {
 		const q = new SlideQueue();
 		const added = q.merge([up('a'), up('pending', { preview: false }), up('t', { thumb: true })], {
diff --git a/frontend/src/lib/diashow/queue.ts b/frontend/src/lib/diashow/queue.ts
index a4cdbba..a0bcf5d 100644
--- a/frontend/src/lib/diashow/queue.ts
+++ b/frontend/src/lib/diashow/queue.ts
@@ -26,14 +26,25 @@ export class SlideQueue {
 	private recentlyShown: string[] = [];
 
 	/**
-	 * Add uploads, de-duplicated by id. Only *displayable* items (preview- or
-	 * thumbnail-ready) are kept — a still-compressing upload has no image yet, so it's
-	 * skipped until a later sync brings it with a preview. `live` items jump onto the live
-	 * queue (shown next); otherwise they join the shuffle pool. Returns the count newly added.
+	 * Add uploads, de-duplicated by id. `live` items jump onto the live queue (shown next);
+	 * otherwise they join the shuffle pool. Returns the count newly added.
+	 *
+	 * Two things are skipped:
+	 *
+	 * 1. **Videos, deliberately and by mime type.** The projector shows stills only — a clip
+	 *    would either hold the slide for its full duration or be cut off mid-way, and it has
+	 *    no audio path on a room's screen. This used to happen only ACCIDENTALLY, as a side
+	 *    effect of the derivative check below: a video whose poster frame extracted fine has
+	 *    a `thumbnail_url` and so was shown as a still frame, while one whose extraction
+	 *    failed was dropped — the same upload included or excluded depending on whether
+	 *    ffmpeg happened to find a frame. Now the rule is the mime type, so it is consistent.
+	 * 2. **Images with no derivative yet.** A still-compressing photo has no image to show;
+	 *    it is picked up by a later sync once its preview exists.
 	 */
 	merge(uploads: FeedUpload[], opts: { live: boolean }): number {
 		let added = 0;
 		for (const u of uploads) {
+			if (u.mime_type.startsWith('video/')) continue;
 			if (!u.preview_url && !u.thumbnail_url) continue;
 			if (this.allKnown.has(u.id)) continue;
 			this.allKnown.set(u.id, u);
diff --git a/frontend/src/routes/diashow/+page.svelte b/frontend/src/routes/diashow/+page.svelte
index 9ea6392..1018217 100644
--- a/frontend/src/routes/diashow/+page.svelte
+++ b/frontend/src/routes/diashow/+page.svelte
@@ -205,6 +205,7 @@
 		// Ids known BEFORE the scan — only these are eviction candidates, so an upload that
 		// arrives mid-scan (not in this snapshot) is never wrongly pruned.
 		const before = new Set(queue.knownIds());
+		// eslint-disable-next-line svelte/prefer-svelte-reactivity -- local scan-bookkeeping set inside one reconcile() call; never stored in $state, so no reactivity is involved.
 		const seen = new Set<string>();
 		let cursor: string | null = null;
 		let complete = false;