How to make are expand instead of disapear

Hi all and thank you for looking at my inquiry :slight_smile:
When I delete the selected lines, the area on top is disappearing instead of expanding to the rouded surface. Any tips on how to do it? what am I doing wrong?

You should be able to trace an edge for the face to form, if it doesn’t then it isn’t flat.
Attaching your model would make it possible to give a fuller answer.

The image appears to be x-ray view, which makes it somewhat difficult to be certain what we are seeing. As @box wrote, if you can share the model we will be able to figure it out.

Here’s a simple example of some of the issues you may be encountering.
GIF 7-05-2024 11-14-48 PM


Great video @Box, I’ll verify that my lines are all well defined.
If I can’t find a solution, @slbaumgartner I’ll post my model!

thank you :slight_smile: