Gst.Uri.get_protocol function is very simple. It just splits string by ":" and return the first part. We can do the same in JS and by doing that we do not have to initialize GStreamer just to get this function.
Show current and toggle change of repeat mode inside the playlist popover. The previous currently playing icon did not reflect actual playing state, so this should be better and does not take more space in UI.
Some titles might be more than few words and will not fit in current playlist popover. Instead of stretching it, show full playlist item filename (or path) on hover in a tooltip.
The button next to the currently playing playlist item acts as a close window button in order to allow closing app without leaving fullscreen. It should use the theme "window-close" icon to better show what it does.
In case of local files the "filename" variable holds the string that is later assigned to the filename property (no difference which is used), but when playing URIs only filename property holds the string. Passing an undefined value to the label caused an error in this case and prevented playback of URIs.
The "clapper_src" directory name was unusual. This was done to make it work as a widget for other apps. Now that this functionality got removed it can be named simply "src" as recommended by guidelines.
