Some operators are working for multiple-object editing however are still not fully benefiting from it.
The most common case is that the operator works but only if all the objects have the same transformation matrix.

• MESH_OT_select_similar (SIMEDGE_FACE_ANGLE, SIMFACE_AREA, SIMFACE_PERIMETER).

Need to create functions equivalent to BM_edge_calc_face_angle, BM_face_calc_area, BM_face_calc_perimeter that can work in the world space, not local space.

• MESH_OT_knife_project and MESH_OT_knife_tool.

Need to convert the code from screen space to world space. More details on D3637.

• UV_OT_weld (UV_STRAIGHTEN, UV_STRAIGHTEN_X, UV_STRAIGHTEN_Y).

The current logic to determine the first and last selected vertex is not very compatible with multi-object. We should be able to select vertices from different objects, and use their position to determine the line extremes.