In the picture there are two views of the expression dialog. One in the sketcher and the other in the property view.
This is specially confusing in the sketcher, as there are two adjacent "OK" buttons, pressing the wrong "OK" will discard any changes done in the expression dialog.
The easy way to resolve this would be to make that dialog modal. There may be better solutions (Modal dialogs are evil).
If there is agreement I will open a case on Mantis.
OS: "Fedora release 28 (Twenty Eight)"
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.14432 (Git)
Build type: Debug
Python version: 2.7.15
Qt version: 4.8.7
Coin version: 3.1.3
OCC version: 6.9.1.oce-0.18
Locale: English/UnitedStates (en_US)