Winforms ScrollContainer not displaying all content, or responding to new content

[freakboy3742]

Reported in #1966.

Sample project:

import toga
from toga.style.pack import Pack, COLUMN


class Quantity(toga.App):
    def startup(self):
        self.count = 1
        self.box = toga.Box(style=Pack(direction=COLUMN))
        main_box = toga.Box(style=Pack(direction=COLUMN))
        for n in range(0, 100):
            label = toga.Label(f"label {n}")
            self.box.add(label)
        main_scroll = toga.ScrollContainer(style=Pack(flex=1))
        main_scroll.content = self.box
        addWidgetsBut = toga.Button("Add Label", on_press=self.butEvent)
        main_box.add(main_scroll)
        main_box.add(addWidgetsBut)
        self.main_window = toga.MainWindow(title=self.formal_name)
        self.main_window.content = main_box
        self.main_window.show()

    def butEvent(self, widget):
        print(f"New label {self.count}")
        label = toga.Label(f"New label {self.count}")
        self.box.add(label)
        self.count += 1


def main():
    return Quantity("App", "org.beeware").main_loop()


main()

On GTK and Cocoa (as of #1969), this app works as expected - all 99 lines of initial content are vertically scrollable, and if new content is added, the new line is appended.

On Winforms (using mainline), only the first ~30 lines of content are displayed, and adding new content seems to remove the vertical scroll entirely.