c# - ReactiveUI - Views not rendering properly (or in full) -

i'm in process of starting new project, using reactiveui , mahapps.metro. first hurdle i've hit problem of views not showing in entirety. have window, , in window have viewmodelviewhost rxui library. simple container nested view-model. activeitem binding binded view-model of user control, , button user control visible on screen.

what expect see window dark gray background , button in middle. see window default background , button in middle. not right. if remove button, see nothing on window, default background.

it seems viewmodelviewhost showing actual contents of usercontrol , disregarding isn't considered real control, such grids etc.

has come across behaviour before?

<mah:metrowindow x:class="...mainwindow"                  xmlns:mah="clr-namespace:mahapps.metro.controls;assembly=mahapps.metro"                  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"                  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"                  xmlns:rx="clr-namespace:reactiveui;assembly=reactiveui"                  windowstartuplocation="centerscreen"                  showtitlebar="false"                  showclosebutton="false"                  showmaxrestorebutton="false"                  showminbutton="false"                  height="768"                  width="1024">     <grid horizontalalignment="center" verticalalignment="center">         <rx:viewmodelviewhost viewmodel="{binding activeitem}" />     </grid> </mah:metrowindow>   <usercontrol x:class="...nestedview"              name="theusercontrol"              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"              xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"               xmlns:d="http://schemas.microsoft.com/expression/blend/2008"              mc:ignorable="d"               textoptions.texthintingmode="auto"              textoptions.textrenderingmode="auto"              d:designheight="768" d:designwidth="1024">      <grid background="darkgray">         <button width="200" height="100" />     </grid> </usercontrol> 

this won't reactiveui problem, simple wpf one.

the viewmodelviewhost centered in window. while usercontrol has designheight , designwidth set, when it's rendered @ runtime automatically size height , width of content - button.

add verticalcontentalignment="stretch" horizontalcontentalignment="stretch" viewmodelviewhost declaration , remove other 2 alignment attributes (they default stretch) , should stretch contents size of window while centering item has fixed size.