"Alpha Channel" field disappeared from the "Select Colour" tab

To highlight a bar or a phrase I always used the method suggested in the Musescore HowTo: https://musescore.org/en/node/220071

1. So first I created a line;
2. Than I set it's colour and thickness in the Inspector (which was later renamed/redesigned to the Properties tab);
3. Finally, to make the line transparent (so that I can see the notes underneath) I set the "Alpha Channel" in Properties/ Appearence/ Colour/ Select Colour tab.

Now I'm on the latest 4.5.1.25... version and it seems to me that the "Alpha Channel" field in Properties/ Appearence/ Colour/ Select Colour tab completely disappeared, the "Alpha Channel" field is gone. So at the moment I just can't find a way to make the line transparent.
See attached:
pic 1- earlier Select Colour
pic 2- recent 5.2 Select Colour (without the "Alpha Channel" field

Any info on where to set transparency/alpha channel, any workarounds or info on why it was removed is appreciated.
Thanks