Tabs — multiple open documents

The tab strip sits between the menu bar and the toolbar. Each tab is an independent document with its own undo/redo history, cells, layers, and view. Click [+] to spawn a blank document, click a tab to switch, click its × (or middle-click) to close.

The clipboard is shared across tabs — copy shapes in tab A, switch to tab B, paste. Shapes paste onto the layer number they came from (a new layer is created if tab B doesn't already have it).