If "Open sidebar" button is clicked while the image is still loading, the sidebar does not appear

Describe the bug
When an image is loading, clicking the "Open sidebar" button will cause the button to disappear and the sidebar to not appear. Sometimes the space for the sidebar is not reserved at all, and sometimes the sidebar leaves leftover space.

To Reproduce
Steps to reproduce the behavior:

1. Click on a photo
2. While it's loading, click the "Open sidebar" button at the top of the photo viewer

Expected behavior
The sidebar appears, either before the photo loads (with placeholder data) or after the photo loads.

Screenshots

Browser logs:

Opening viewer for file  (filename censored) viewer-main.js:374:3626
A files list have been provided. No folder content will be fetched. viewer-main.js:374:5798

(Also, earlier on, while loading the gallery I also get these, but this might be unrelated:)

The following mime is already registered image/svg+xml 
Object { id: Getter & Setter, group: Getter & Setter, mimes: Getter & Setter, component: Getter & Setter, … }
viewer-main.js:374:7723
The following mime is already registered text/plain 
Object { id: Getter & Setter, mimes: Getter & Setter, component: Getter & Setter, group: Getter & Setter, … }
viewer-main.js:374:7723
The following mime is already registered text/csv 
Object { id: Getter & Setter, mimes: Getter & Setter, component: Getter & Setter, group: Getter & Setter, … }
viewer-main.js:374:7723

System info:

• OS: Arch Linux
• Browser: Firefox 81.0.1
• Nextcloud version: 19.0.4

Reproduce the issue in the photo viewer by opening a photo and clicking “Open sidebar” while it loads; the report includes viewer-main.js browser logs and screenshots. Trace the loading and sidebar interaction from the viewer entry point, then verify that the sidebar appears and reserves the correct space before or after the photo finishes loading.