Controls

Change alpha: q and e
Zoom: scroll.
Pan: w, a, s, d
Polygon creation:
Press ctrl to enter create mode. In this mode the mouse pointer is a crosshair.
Left click to add new vertices
Right click to close current polygon.
z to undo the last click
esc to delete the entire current polygon and return to select mode.
Vertex manipulation:
Move vertex: select polygon, then click & drag on vertex
Delete vertices: select polygon, then press and hold z and click on vertex
Add vertex: select polygon, then press and hold shift and click on edge
Polygon and instance manipulation:
Select polygon: click inside polygon. Or expand the polygon group in the side bar and click on the polygon entry.
Select polygon group: click on group entry in sidebar. Click on the triangle at the end to expand/collapse group.
Clear selection: press esc to clear the current selection.
Change color: select polygon/polygon group, click change color button on top of sidebar. This changes color for the entire instance.
Delete: select polygon/instance, press delete or backspace key to delete the entire polygon.
Change class: select polygon/instance, select new class label with the selector on top of sidebar.
Change visibility order: select single polygon (by clicking on canvas or sidebar). Use + or - to adjust the ordering (bring forward/back one layer at a time). Use pageUp/pageDown to bring polygon to top or bottom of the hierarchy.
Split polygon: select polygon, then press and hold x, first click on an edge, and then click inside the polygon, finish the path with another click on an edge.
Merge polygons: select polygon (optional), press and hold c, click on all other parts (separate polygons), then release c.