I have tried debugging functionality of the application “KDevelop 5.2.4-43.3” out. I looked at a few variables. Corresponding values were displayed from the debug session in a popup window (or in a paned window). I have noticed then that indications (symbol “>”) for the unfolding of further items were given. But the symbol was changed to “├” after I clicked on interesting data structure members.
It seems that this happens for a class like “QString” so far. (May I drill down into such a data structure?)

https://docs.kde.org/trunk5/en/extragear-kdevelop/kdevelop/debugging-programs-in-kdevelop.html#running-a-program-in-the-debugger