background color ignored by sub box label widget

[BrendanSimon]

If I set a box with a background color, label widget children of that box are displayed correctly with the specified background.

However, if a child is another box that contains a label, then the background colour of the outer box is not propagated to the inner boxes.

Is that the expected behaviour?

I would have thought the I could set a background colour of a box and then all sub-widgets (no matter how deep in sub-boxes) would also be rendered with the same background colour.

#!/usr/bin/env python

import toga
from toga.style import Pack
from toga.style.pack import ( COLUMN, ROW, CENTER )

#!============================================================================

class Test_App( toga.App ) :

    def startup( self ) :
        #! Create the main window
        self.main_window = toga.MainWindow( title=self.name )

        body_box = toga.Box( id = 'days_box',
                             style = Pack( direction=ROW, flex=0, background_color='yellow' ),
                             children = [
                                toga.Label( "XXX" ),
                                toga.Box( children = [ toga.Label( "YYY" ) ] ),
                                toga.Label( "ZZZ" ),
                                        ],
                             ) 

        main_box = toga.Box( id = 'main_box',
                             style = Pack( direction=COLUMN, flex=0 ),
                             children = [ body_box ],
                             )

        self.main_window.content = main_box

        #! Show the main window
        self.main_window.show()

#!============================================================================

def main() :
    return Test_App( 'Test App', 'au.com.etrix.test_app' )

Note how label "YYY" does not have the specified background colour. It has the default system colour.

[freakboy3742]

Agreed this is a little unexpected. I can certainly explain it - but that explanation is more of a rationalization than a satisfying API.

The underlying problem here is that styles don't currently cascade (or, more specifically, there's no concept of "inherit" as a value in Pack).

Broadly, we should be aiming to follow the lead of HTML and CSS here; so the YYY block should be inheriting it's color from it's parent. In this case, it's, not because it's getting the default background color.

[freakboy3742]

Doing some housekeeping; it occurs to me that one simple fix here might be to make the default background color for box transparent/no fill - that way, a child box will inherit the parent color by virtue of being transparent.

[mhsmith]

Only some properties are inherited in CSS, and background-color isn't one of them. However, CSS's default background-color is indeed transparent. So I think that's a better solution.