Hi osx59 - actually that isn't a bug. When you selected the second set of lines, those are the edges of the trimmed surface. When all edges of a trimming boundary are selected and you then hit delete, you "untrim" it and recover the underlying surface.

There is some more information on how "trim curves" and "underlying surfaces" work here which may be helpful.

Also here is an earlier message that shows how untrimming can be useful - here I show how you can remove trims to "repair" cuts and holes that you previously placed on an object, such as to erase a window:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4


You can also see what is happening if you select the surface after you initially create it and then use Edit/Show pts to turn on the surface's control points. That will show you that the underlying surface is that larger rectangle.

The Planar command always creates a trimmed surface from the curves with the surface at a bit larger size, the larger size can sometimes help a bit in calculating intersections in certain special cases.

There are a few different options for you - first you can just avoid untrimming the surface if you don't want that to happen. (just don't hit delete that second time)

Second, you can use the Draw solid / Plane command to draw your plane object instead of Construct / Planar - the "Draw" command will create a plane with an underlying surface that is the same as the edge boundaries. This also has the benefit of taking fewer steps because you don't have to create the surface separately and then delete the curves afterwords, you just create the plane directly all at once with no extra pieces involved.

Lastly, you can set up a keyboard shortcut with the "ShrinkTrimmedSrf" command on it - if you select your surface and then run ShrinkTrimmedSrf on it by pushing the key for it, it will then shrink the underlying surface down until it hits the trimming boundaries. That is a way you can edit the trimmed plane to work more in the way that you expected.

- Michael