electrum/gui/kivy/uix/ui_screens/amount.kv

<KButton@Button>:
    size_hint: 1, None
    height: '38dp'

Popup:
    id: popup
    title: _('Amount')

    BoxLayout:

        orientation: 'vertical'
        
        halign: 'center'

        Label:
            id: amount_label
            text: ''

        GridLayout:
            cols: 3
            size_hint: 0.5, 1
            KButton:
                text: '1'
                on_release: amount_label.text += self.text
            KButton:
                text: '2'
                on_release: amount_label.text += self.text
            KButton:
                text: '3'
                on_release: amount_label.text += self.text
            KButton:
                text: '4'
                on_release: amount_label.text += self.text
            KButton:
                text: '5'
                on_release: amount_label.text += self.text
            KButton:
                text: '6'
                on_release: amount_label.text += self.text
            KButton:
                text: '7'
                on_release: amount_label.text += self.text
            KButton:
                text: '8'
                on_release: amount_label.text += self.text
            KButton:
                text: '9'
                on_release: amount_label.text += self.text
            KButton:
                text: '.'
                on_release: amount_label.text += self.text
            KButton:
                text: '0'
                on_release: amount_label.text += self.text
            KButton:
                text: '<'
                on_release: amount_label.text = amount_label.text[:-1]

        BoxLayout:
            size_hint: 0.5, None
            Button:
                size_hint: 0.5, None
                height: '48dp'
                text: _('Max')
                on_release: popup.dismiss()
            Button:
                size_hint: 0.5, None
                height: '48dp'
                text: _('OK')
                on_release: popup.dismiss()

        Widget:
            size_hint: 1, None