avoid that HSplit produces gaps between children

hi @jonathanslenders,
i'd appreciate if you could give me a hint how to produce the desired effect.

from __future__ import unicode_literals
from prompt_toolkit.layout.containers import HSplit, VSplit, Window, ScrollOffsets, VerticalAlign
from prompt_toolkit import Application
from prompt_toolkit.layout.controls import FormattedTextControl
from prompt_toolkit.key_binding import KeyBindings
from prompt_toolkit.layout.dimension import LayoutDimension, Dimension
from prompt_toolkit.layout.layout import Layout
from prompt_toolkit.styles import Style

def gen_win(name, lines):
	def get_text_fragments():
		tokens = []
		tokens.extend([('class:blah.title', name + '\n')])
		for i in xrange(lines):
			tokens.append(('class:blah', '#####################'))
			tokens.append(('class:blah', '\n'))
		tokens.pop()
		return tokens
	return Window(
		FormattedTextControl(get_text_fragments),
		style='class:blah',
		width=Dimension.exact(16),
		height=Dimension(min=3),
	)

cont = HSplit([
	gen_win('foo', 3),
	gen_win('bar', 9),
], align=VerticalAlign.TOP)

kb = KeyBindings()
@kb.add(u'q')
@kb.add(u'c-q')
def exit_(event):
	event.app.exit()

app = Application(
	layout = Layout(
		cont,
	),
	full_screen=True,
	key_bindings=kb,
	style = Style.from_dict(
		{
			'blah':       'bg:#bbbbbb #000000',
			'blah.title': 'bg:#668866 #ffffff',
		}
	),
)

app.run()

what i'd like is basically

foo
##########
##########
##########

bar
##########
##########
...

<empty space here>

but what i get is

foo
##########
##########
##########

<empty space here>

bar
##########
##########
...

Start by running the issue's Python reproducer and inspect how HSplit, VerticalAlign.TOP, Window, and Dimension determine child heights. Confirm the layout behavior against the shown output, then use the relevant layout tests or add a focused regression test. Done means the unused space appears after the children rather than between them.