Hi wim, I think that might be a bug but unfortunately probably a difficult one to fix. The curve/curve booleans can tend to get somewhat confused by too many overlapping pieces. Or in this case it may be getting confused about properly grouping the correct outlines with each other.
If the boolean command does not produce the correct result for you, you can instead use the Edit > Trim command and that will give you more detailed control about picking which individual cut up pieces should be discarded.
So for your case here, to use Trim select these 2 rectangles:
Now run the Edit > Trim command. At the prompt to select cutting objects, just right-click or push done at that stage since you want to do what's called "mutual trim" where each object cuts the other, rather than selecting separate cutting objects.
At this stage in the Trim command it will now show you the cut up fragments:
You can now select these fragments as the pieces to discard, do a window select to grab each section, starting with the window on the left and dragging towards the right which will capture things only contained totally inside the window. If you do it from right towards left then it captures anything that touches the window. So you want to make a selection like this:
Then right-click to end the Trim command and you'll have this result:
If the curve booleans get confused in any particular situation you should be able to use the Trim command instead in this type of way to still get the result you need. It may take a couple of extra picks to do it, so it's not a bad idea to try the boolean way first.
You can actually get the curve booleans to work here though if you have somewhat less overlap of the pieces, by using a boolean difference instead of a union. To do that, select just this inside frame part alone (not the outside rectangle at all):
Then run Construct > Boolean >
Difference (not Union), and select the little one as the the cutting object:
That will then make this result:
Hope this helps!
- Michael
