Pretty simple to duplicate the problem I'm having. create a Popup node, add an HBoxContainer to it and a TextureRect to it, tiling the default icon.png. set all size flags to fill/expand, size the popup to the screen (my project is using disabled stretch settings), and of course call popup in the popup's _ready().
The questions are:
Why doesn't the HBox fill the popup?
Why do some control nodes have size flags if they are ignored?
If I'm not mistaken it wouldn't take more than a dozen lines to code the behavior I'm looking for, but then I would feel like I'm reinventing the wheel and not using the engine's built in methods/properties causing me to use a solution that may be less optimized than already exists.