> Etienne, what version are you using?
I was about to install XUbuntu in a VM when I suddenly found the cause.
When the viewer cannot create a WebGL context (or if the "Legacy viewer" checkbox is checked when WebGL is supported) the viewer draws with a second code, which is simplified to work with more browsers.
I did not update the legacy viewer with the new modes, and observed your behavior when checking the checkbox.
I will try to find time to fix this.