Today, when a user enters a property value, the inspector check its validity, and set back the previous value if the entered value is invalid.
This is problematic, since SB erase data entered by a user.
The worst case is for "complex" values such as Style.
e.g drop a Button, and from Properties section, enter "-fx-background-color," (syntax error : ',' instead of ';'). This is a CSS syntax error, the value is erased, and an error message appears in the message panel.

We also have the case of incorrect values, accepted by the API. For instance, negative height or width. This is accepted by the Fx API, but have unexpected effect (see DTL-4991). SB should warn the user in this case.