I've only had PTE AV Studio 10 for a couple of months, but have spent quite a few hours banging around within (& have learned a LOT. Really nice design & terrific capabilities). Nonetheless, I still struggle w/ filling the text box w/ a semi-transparent, solid color background. In my initial post I comment that a semi-transparent text background IS ACHIEVABLE following the stipulated rules, but it seems tortured for something that would seem to be a common function. Most of the time it works, but occasionally the choice of 100% opaque is insufficient for the image over which the text appears. Of course, the settings are for Drop Shadow & Glow, so I understand why the PTE designer might want to keep a shadow from being fully opaque.
So I tried the suggestion in your reply of April 11. Maybe I'm missing something, but this approach is even more tortured. My guess is that the 1st step is to create the text so the user can then create a rectangle of the right size. Step 2: insert rectangle. Step 3: Go to Properties/Change (Vertical) Native Resolution so rectangle height is same as text. Step 4: change (Horizontal) native resolution to width of text. No wait, "Fit mode" (Fit or Cover) constrains that. So, go to "Animation", unchain/unlock X&Y Zoom. Adjust Y-Zoom to height of text (but don't grab the handles on the rectangle image, or X & Y are again zooming together). Step 5: position rectangle over the text & move it underneath (Ctrl+Page Down). But don't make it a "child" of the rectangle or its size will change. OR, start 1st w/ an approximate sized rectangle & insert text as a "child" (as you suggest). Same problems w/ native resolution & "Fit", though workarounds similar to the above discussion will bring the rectangle to a good size, but the text inside bears little relationship to text elsewhere in the slideshow (text zoom of 15 here may be way different from text created w/o the "child of a rectangle".
Granted, I'm little more than a newbie & maybe it's easier than I'm making it. And I'm asking myself this question: If I believe this is a common function that needs the software designers' attention, why has no one else mentioned it?