Display never re-shows a slide's content if its id is unchanged, even when the HTML changed #21

New issue

Closed

opened 2026-08-13 20:06:47 +00:00 by paulloth1 · 1 comment

paulloth1 commented

2026-08-13 20:06:47 +00:00

(Migrated from github.com)

Problem

display.html's applySlide() (around line 104-112) dedups purely on slide id:

function applySlide(data) {
  if (data.id === lastId) {
    return; // same slide already showing, don't touch the DOM (avoids img/video reload flicker)
  }
  ...
}

This is correct for avoiding flicker within a single slide's display window (the backend holds the same id + html for the whole rotation_interval_seconds, and the client polls every 5s — re-applying identical HTML every poll would be wasteful/flickery).

But backend/rotation.py's _rotation_step() re-renders every enabled slide from scratch on every lap of the rotation loop (shown_any/set_current around line 139-159) — so a slide with dynamic content (RSS feed, weather widget, departure board, anything server-rendered) gets genuinely new html on its next lap, while keeping the exact same id. The client-side dedup has no way to tell "same id, same content" apart from "same id, refreshed content" — it always takes the fast path and never updates the DOM.

With the site's current 10-slide rotation this is masked, since many different ids cycle through between two occurrences of the same slide, resetting lastId in between. But it's a real bug that will bite:

Any deployment with only 1-2 enabled slides (the same id could plausibly be polled twice in a row across a lap boundary).
Even in the current 10-slide setup, it's fragile — disabling slides down to just the RSS ones, or any future single-slide "ticker mode" use case, would silently freeze the display on stale content forever (rotation keeps re-rendering server-side, but the client-visible screen never updates).

Suggested fix

Compare data.html (or a cheap hash of it) in addition to data.id — only short-circuit when both are unchanged from last time. Something like:

if (data.id === lastId && data.html === lastHtml) {
  return;
}

(lastHtml is already tracked, just not consulted in this check.)

Where

backend/templates/display.html:104-112

## Problem `display.html`'s `applySlide()` (around line 104-112) dedups purely on slide id: ```js function applySlide(data) { if (data.id === lastId) { return; // same slide already showing, don't touch the DOM (avoids img/video reload flicker) } ... } ``` This is correct for avoiding flicker *within* a single slide's display window (the backend holds the same `id` + `html` for the whole `rotation_interval_seconds`, and the client polls every 5s — re-applying identical HTML every poll would be wasteful/flickery). But `backend/rotation.py`'s `_rotation_step()` re-renders every enabled slide from scratch on every lap of the rotation loop (`shown_any`/`set_current` around line 139-159) — so a slide with dynamic content (RSS feed, weather widget, departure board, anything server-rendered) gets genuinely new `html` on its next lap, while keeping the exact same `id`. The client-side dedup has no way to tell "same id, same content" apart from "same id, refreshed content" — it always takes the fast path and never updates the DOM. With the site's current 10-slide rotation this is masked, since many different ids cycle through between two occurrences of the same slide, resetting `lastId` in between. But it's a real bug that will bite: - Any deployment with only 1-2 enabled slides (the same id could plausibly be polled twice in a row across a lap boundary). - Even in the current 10-slide setup, it's fragile — disabling slides down to just the RSS ones, or any future single-slide "ticker mode" use case, would silently freeze the display on stale content forever (rotation keeps re-rendering server-side, but the client-visible screen never updates). ## Suggested fix Compare `data.html` (or a cheap hash of it) in addition to `data.id` — only short-circuit when *both* are unchanged from last time. Something like: ```js if (data.id === lastId && data.html === lastHtml) { return; } ``` (`lastHtml` is already tracked, just not consulted in this check.) ## Where `backend/templates/display.html:104-112`

paulloth1 commented

2026-08-13 20:13:14 +00:00

(Migrated from github.com)

Fixed and deployed. applySlide() in display.html now short-circuits only when both id AND html are unchanged. Verified the fix doesn't regress first-load (lastId starts undefined) or the iframe-reset path (calls showHtml directly, bypassing applySlide entirely). kiosk.service restarted since this touches the template the kiosk browser loads once at startup.