Button to close the preview panel does not appear for certain messages containing a URL

[psrpinto]

Steps to reproduce

1. Under Settings -> Preferences -> Images, GIFs and videos, turn on the toggle for Enable inline URL previews by default
2. Send the following message (without quotes): "https://matrix.org/docs/spec/client_server/latest https://matrix.org"
3. Place your cursor on the URL preview that appears after you sent the above message

What did you expect?

Hovering the preview panel should make the button (X) to close it appear.

What happened?

The button (X) to close the preview panel does not appear when hovering the preview panel.

The image below illustrates the issue. On the left, the button is correctly displayed, on the right it isn't:

Operating system

macOS Big Sur

Browser information

Firefox 92.0

URL for webapp

app.element.io

Homeserver

matrix.org

Have you submitted a rageshake?

No

[psrpinto]

The issue is caused by the preview of the first URL being empty (i.e. the call to /media/r0/preview_url returns {}). This causes the render() method of LinkPreviewWidget to return early.

Since the close button is passed to the first LinkPreviewWidget, and we returned early, it won't be rendered.

[germain-gg]

Thank you for raising this issue and investigating it too!
We'd welcome contributions if you're interested in giving it a go