This is about wxPython.
I would like to have 2 Panels laying one over the other:
PanelBG should be some sort of a "background", with its own GridBagSizer with subPanels, StaticTexts and so on;
PanelFG should be the "foreground" panel, also with its own GridBagSizer with some StaticTexts, Buttons... but a transparent background, in such a way that PanelBG is visible wherever PanelFG doesn't lay widgets.
I need both Panels to stretch to all the sides of the frame, even when resizing the window, though never changing the reciprocal proportions, that's why I'm not sure if there's a way to use absolute positioning.
In case you are wondering, the reason why I don't want to use a single Panel is that merging the 2 GridBoxSizers would require me to place many many more cells in the sizer, because rows and columns of foreground and background don't always coincide, and I should split them in many cells, with grid dimensions growing up to hundreds**2.
Since the content I want to put in the foreground needs to be updated and refreshed quite often, this would require redrawing all the cells every time, which would take 10 - 20 seconds to complete the operation (tested). Updating only the foreground would require just some hundredths of a second instead.
Thank you!
This would be at least partially a change of direction, but it might be worth examining what other rendering options you have. 
In particular, I'm thinking of wxWebKit (http://wxwebkit.kosoftworks.com/), which would let you do layering, etc. using the WebKit browser rendering engine. I'm not sure whether it's at a stage that would provide everything you need since I haven't actually used it, but even if it doesn't work then it may be an approach worth trying - using HTML/CSS for part of your display, while wrapping the whole in a wxPython app.
wxPython: how to lay one panel over another