Clicking on a layout background commits the record and thus validates all the fields. That's what's happening, now to prevent it from happening.
This sounds a bit weird, but it works.
Use the Webviewer tool to place an empty, invisible web viewer on your layout. Size it to cover the entire body layout part and move it to the back so that it is behind all other layout objects. This object will intercept those pesky background mouse clicks and keep them from committing the record. (To make it empty and invisible, you delete the viewer's URL expression and give it either a transparent border pattern or a border with line width of zero.)