Hemant Vishwakarma: Why do empty lines in Gtk.TextView misalign with line numbers but lines with text don't?

I have a Gtk application written in Python 3, and I want to make a line numbers widget. My current tactic is to use two Gtk.TextViews in a Gtk.HBox which is in a Gtk.ScrolledWindow. I have a css theme loaded that sets the TextViews' font to monospace, but there's a problem.

When I type lines of text in the main TextView, the line numbers line up with the text I'm typing. However, when I type a bunch of empty lines, the line numbers gradually get misaligned with the text.

Here's the code I'm using:

import gi

gi.require_version("Gdk", "3.0")
gi.require_version("Gtk", "3.0")
from gi.repository import Gdk, Gtk

class Window(Gtk.Window):

    def __init__(self):
        Gtk.Window.__init__(self)
        self.connect("delete-event", Gtk.main_quit)

        self.sw = Gtk.ScrolledWindow()
        self.add(self.sw)

        self.sw_box = Gtk.HBox()
        self.sw.add(self.sw_box)

        self.t1 = Gtk.TextView()
        self.t1.do_insert_at_cursor(self.t1, "1\n" * 50)
        self.t1.set_editable(False)
        self.sw_box.pack_start(self.t1, False, False, 0)
        
        self.t2 = Gtk.TextView()
        self.sw_box.pack_start(self.t2, True, True, 0)

        self.load_theme()

        self.show_all()

    def load_theme(self):
        """Load the theme."""
        provider = Gtk.CssProvider.new()
        display = Gdk.Display.get_default()
        screen = display.get_default_screen()
        Gtk.StyleContext.add_provider_for_screen(screen, provider, Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION)
        provider.load_from_path("theme.css")

if __name__ == "__main__":
    w = Window()
    Gtk.main()

theme.css

* {
    background-clip: padding-box;
    -GtkToolButton-icon-spacing: 4;
    -GtkTextView-error-underline-color: #FC4138;
    -GtkScrolledWindow-scrollbar-spacing: 0;
    -GtkToolItemGroup-expander-size: 11;
    -GtkWidget-text-handle-width: 20;
    -GtkWidget-text-handle-height: 20;
    -GtkDialog-button-spacing: 4;
    -GtkDialog-action-area-border: 0;
    outline-color: alpha(currentColor,0.3);
    outline-style: dashed;
    outline-offset: -3px;
    outline-width: 1px;
    font-family: 'Trebuchet MS', 'Lucida Sans Unicode', 'Lucida Grande', 'Lucida Sans', Arial, sans-serif;
    -gtk-outline-radius: 2px;
}

.gtkstyle-fallback:selected {
    background: #ff00ff;
}

.background {
    color: #D3DAE3;
    background: #000000;
}

*:selected {
    color: #000000;
    background-color: #ffff00;
}
*:disabled {
    -gtk-icon-effect: dim;
    background: #000000;
}

button {
    background-image: none;
    border-color: #000000;
}
button:active {
    border-color: #ffff00;
}

menuitem {
    background: #000000;
    box-shadow: none;
    border: 1px solid;
    border-color: #000000;
}
menuitem:hover {
    color: #ffffff;
    border: 1px solid;
    border-color: #ffff00;
}

messagedialog .titlebar {
    background: #000000;
}

notebook tabs{
    background: #000000;
}
notebook tab:checked {
    border: 1px solid;
    border-color: #ffff00;
    box-shadow: none;
}

scrollbar slider {
    background-color: #aaaa00;
}
scrollbar slider:active {
    background-color: #ffff00;
}

scrolledwindow {
    color: #ffffff;
    border: 1px solid;
    border-color: #ffff00;
}

textview {
    font-family: monospace;
}
textview text {
    color: #ffffff;
    background-color: #000000;
}
textview text selection {
    color: #000000;
    background-color: #ffffff;
}