Paints the background of the glyph window when you are not in the foreground

show-layer-color-in-background.py

from mojo.subscriber import Subscriber, registerGlyphEditorSubscriber, unregisterGlyphEditorSubscriber


class ShowLayerColorInBackground(Subscriber):
    
    debug = True
        
    def build(self):
        glyphEditor = self.getGlyphEditor()
        
        container = glyphEditor.extensionContainer(
            identifier="com.arialcrime.ShowLayerColorInBackground",
            location="background",            
            clear=True
        )
        self.backgroundLayer = container.appendRectangleSublayer(
            opacity=0.55,   
            visible=False
        )
    
    def roboFontDidSwitchCurrentGlyph(self, info):
        g = info["glyph"]
        if g is not None:
            font = g.font
            layer = g.layer
            with self.backgroundLayer.propertyGroup():
                self.backgroundLayer.addSkewTransformation(-(font.info.italicAngle or 0))
                self.backgroundLayer.setPosition((font.lib.get("com.typemytype.robofont.italicSlantOffset", 0), font.info.descender))  
                self.backgroundLayer.setSize((g.width, font.info.unitsPerEm))    
                self.backgroundLayer.setFillColor(g.layer.color)
                self.backgroundLayer.setVisible(g.font.defaultLayer != g.layer)
        
    def glyphEditorLayerDidChangeColor(self, info):
        layer = info["layer"]
        self.backgroundLayer.setBackgroundColor(layer.color)
    
    def glyphEditorGlyphDidChangeMetrics(self, info):
        g = info["glyph"]    
        font = g.font
        self.backgroundLayer.setSize((g.width, font.info.unitsPerEm))
        

registerGlyphEditorSubscriber(ShowLayerColorInBackground)

dont check default layer by its order in the glyph.layers

g = CurrentGlyph()

print(g.font.defaultLayer, g.layer)
print(g.font.defaultLayer == g.layer)

some adjustments, mainly to make a lot easier :)

from mojo.subscriber import Subscriber, registerGlyphEditorSubscriber, unregisterGlyphEditorSubscriber


class ShowLayerColorInBackground(Subscriber):
    
    debug = True
        
    def build(self):
        glyphEditor = self.getGlyphEditor()
        
        container = glyphEditor.extensionContainer(
            identifier="com.arialcrime.ShowLayerColorInBackground",
            location="background",            
            clear=True
        )
        self.backgroundLayer = container.appendRectangleSublayer(
            opacity=0.55,   
            visible=False
        )
    
    def roboFontDidSwitchCurrentGlyph(self, info):
        g = info["glyph"]    
        font = g.font
        layer = g.layer
        with self.backgroundLayer.propertyGroup():
            self.backgroundLayer.addSkewTransformation(-font.info.italicAngle)
            self.backgroundLayer.setPosition((0, font.info.descender))  
            self.backgroundLayer.setSize((g.width, font.info.unitsPerEm))    
            self.backgroundLayer.setFillColor(g.layer.color)
            self.backgroundLayer.setVisible(g.font.defaultLayer != g.layer)
        
    def glyphEditorLayerDidChangeColor(self, info):
        layer = info["layer"]
        self.backgroundLayer.setBackgroundColor(layer.color)
    
    def glyphEditorGlyphDidChangeMetrics(self, info):
        g = info["glyph"]    
        font = g.font
        self.backgroundLayer.setSize((g.width, font.info.unitsPerEm))
        

registerGlyphEditorSubscriber(ShowLayerColorInBackground)

Oh wow, thanks for the edits. That is indeed much cleaner. 😅