QuickDesigner: Alpha spinner's button are incorrectly highlighted

Steps to reproduce:

• open/create Qt Quick UI or Application
• select an Item where you can change the color property
• click the color preview to open the color chooser, select a color

Current result:

• the alpha channel is set to 1.0, the spinner up button is displayed as enabled, the spinner down button is displayed as disabled
• both spinner buttons still work as expected (it's not possible to use the up button as 1.0 is maximum, but you can use the down button - so, it's only the display which is wrong)