Viewlet is a tab in the left area in VS Code. Active means visible in this context.
In VS Code there can be only one visible viewlet at any time. It is not true for Theia, since views can be layed-out again to different areas. So only last visible view will be an active viewlet.
Panel is a tab in the bottom area in VS Code. Active means visible in this context.
In VS Code there can be only one visible panel at any time. It is not true for Theia, since views can be layed-out again to different areas. So only last visible view will be an active panel.