What steps will reproduce the problem?
1.  Load BetterBlizzOptions
2. Open the Interface options for any addon
3. Click the "Defaults" button in the bottom left-hand corner
4. Notice the confimation dialog pops up "behind" the InterfaceOptionsFrame, where it cannot be clicked.

What is the expected output? What do you see instead?
I expect StaticPopup confirmation dialogs launched from InterfaceOptionsFrame to appear above it, not beneath it. The same problem happens with any addon-specific StaticPopup_Show() commands launched from InterfaceOptionsFrame (eg by an AceConfig execute button).

What version of the product are you using?
r39-release

Please provide any additional information below.

The problem is caused by LibBetterBlizzOptions-1.0.lua:132

InterfaceOptionsFrame:SetFrameStrata("FULLSCREEN_DIALOG")

which changes the default InterfaceOptionsFrame strata from "HIGH" (which is below StaticPopup that runs at "DIALOG") to "FULLSCREEN_DIALOG" which is above StaticPopup.

I don't see a good reason for the library to messing with the InterfaceOptionsFrame strata, especially in light of the fact that it breaks StaticPopup behavior.
The fix is to remove that line.