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WPF listbox binding selected only updates items in view

I've been running into a strange issue with a listbox within a UserControl with WPF and MVVM. The listbox has its items bound via a ViewModel and an ObservableCollection<T>, the style property IsSelected is bound to a property Selected inside the individual items.
[...]
<ListBox ItemsSource="{Binding Path=SQLObjects}" FontFamily="Consolas" Margin="5,0,5,5" DisplayMemberPath="Name" SelectionMode="Extended" SelectedItem="{Binding Path=SelectedSQLObject}">
<ListBox.Resources>
<Style TargetType="{x:Type ListBoxItem}" BasedOn="{StaticResource MetroListBoxItem}">
<Setter Property="IsSelected" Value="{Binding Selected, Mode=TwoWay}" />
</Style>
</ListBox.Resources>
</ListBox>
[...]
When a user selects a single or multiple items (with ctrl or shift) the properties are updated as expected. But selecting all items with ctrl + a only updates the property Selected of the items visible inside the scroll area of the listbox.
in the screenshot all items are selected with ctrl + a but only the items currently visible have their property updated. When scrolling down in the listbox without changing the selection, all items are updated as they come into view.
Could this be caused by MahApps.Metro?
The ViewModel is bound to the UserControls Datacontext inside the xaml in the main window.
[...]
<TabItem Header="Trigger">
<userControls:ObjectControl>
<userControls:ObjectControl.DataContext>
<viewmodel:TriggerObjectViewModel DialogCoordinator="{x:Static local:MainWindow.Coordinator}" />
</userControls:ObjectControl.DataContext>
</userControls:ObjectControl>
</TabItem>
[...]
All other TabItems use the same usercontrol and have the same issue.
I would like to not use a workaround (e.g. scrolling trought the whole list when the user selects items outside of the current view), is there any kind of property I could set for WPF to update all items inside a collection, regardless if they are visible or not?

I suspect this is due to virtualization. The UI items don't exist until they are scrolled into view.
Try turning off virtualization on your list control:
<ListBox ItemsSource="{Binding Path=SQLObjects}"
VirtualizingPanel.IsVirtualizing="False">

WPF:Difference between TabControl.ItemTemplate and TabItem.ContentTemplate

I'm confused on this for a long time,these both seem to affect the tabitems' presentation in the tabcontrol.
Is it designed for best control of the presentation of the tabcontrol?
Or if there is something I dont't understand.

There's some very long answers here for what is actually a very simple question. To avoid confusion:
ItemTemplate is the template used to format each item in the ItemsSource to create the headers (the controls that appear in the tab bar) and ContentTemplate is the template used to format each item in the ItemsSource to create the content of the tabs (the controls that appear when you click on the header).

The ItemsControl.ItemTemplate Property is used to define what each item in the data bound collection should look like... from the ItemsControl.ItemTemplate Property page on MSDN:
Gets or sets the DataTemplate used to display each item.
As you can see, it is of type DataTemplate, which is customary for a template that displays data... its DataContext will automatically be set to an item from the collection and so controls declared in that DataTemplate will automatically have access to the items properties. Please see the Data Templating Overview page on MSDN for further help with this.
Similarly, from MSDN, the ContentControl.ContentTemplate Property:
Gets or sets the data template used to display the content of the ContentControl.
Again, its DataContext will automatically be set to the object that is set as the Content property. Please note that the ContentControl only has a ContentTemplate Property and no ItemTemplate Property, which is used for collection items... from the Data Templating Overview page on MSDN:
Because myTaskTemplate is a resource, you can now use it on other controls that have a property that takes a DataTemplate type. As shown above, for ItemsControl objects, such as the ListBox, it is the ItemTemplate property. For ContentControl objects, it is the ContentTemplate property.
UPDATE >>>
To clarify this situation further, think of this simple rule:
Use the ContentTemplate property to define how an object that is set as the Content property of a ContentControl should look.
Use the ItemTemplate property to define how the items of a collection control should look.
That the difference at its simplest. However, I'd like to point out that as these properties are both of type DataTemplate, their values are interchangeable.
For example, let's say that you have a Person class and you display a collection of Person objects in a ListBox. You can declare a DataTemplate to set as the ListBox.ItemTemplate property to define how each Person in the collection should look. However, if you just wanted to display a single Person, then you could use a ContentControl with the Content set to an instance of the Person class, and still use the same DataTemplate, but set as the ContentTemplate instead:
Multiple objects:
<ListBox ItemsSource="{Binding People}" ItemTemplate="{StaticResource Template}" ... />
...
Single object:
<ContentControl Content="{Binding Person}"
ContentTemplate="{StaticResource Template}" ... />

Setting the TabControl.ItemTemplate you specify a template to use for all TabItems in the Items collection of the TabControl, unless you override the TabItem.ContentTemplate for a specific TabItem.
So, while they do the same, TabControl.ItemTemplate is a more generic template for all the TabItems in the TabControl and TabItem.ContentTemplate is specific for the TabItem it is used in.
The above is not quite true, as TabControl has an ItemTemplate property and a ContentTemplate property, to make it more confusing.
ItemTemplate is used as template for the header (the tab thingy) of all TabItems added through databinding on the ItemsSource or through Xaml without making the the added item a TabItem:
<TabControl ItemsSource="{Binding ListOfItems}">
<TabControl.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding}" Foreground="Red"/>
</DataTemplate>
</TabControl.ItemTemplate>
<TabControl.ContentTemplate>
<DataTemplate>
<TextBlock Text="{Binding}" Foreground="Blue"/>
</DataTemplate>
</TabControl.ContentTemplate>
</TabControl>
This will create a TabControl with red text in the header/tab and blue text for content.
Now, if we do the following:
<TabControl>
<TabControl.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding}" Foreground="Red"/>
</DataTemplate>
</TabControl.ItemTemplate>
<TabControl.ContentTemplate>
<DataTemplate>
<TextBlock Text="{Binding}" Foreground="Blue"/>
</DataTemplate>
</TabControl.ContentTemplate>
<TabItem Header="One" Content="One"/>
<TabItem Header="Two" Content="Two"/>
<TabItem Header="Three" Content="Three"/>
</TabControl>
We'll have a TabControl with three tabs, and the header text is black, content is still blue. And a DataError informing us that the ItemTemplate and ItemTemplateSelector properties are ignored for items already of the ItemsControl's container type, in this case TabItem. In this case, we need to specify TabItem.HeaderTemplate to change the appearance of the header.
So TabControl.ItemTemplate and TabItem.ContentTemplate don't do the same, but my previous explanation still holds for TabControl.ContentTemplate and TabItem.ContentTemplate.

Why does this ComboBox ignore the DataTemplate when SelectedItem is a ContentControl?

In our application we have a screen design feature which is comprised of a custom ScreenDesignPanel and a Property Grid with a ComboBox at the top which points to the selected item on the ScreenDesignPanel. This allows the user to select the UIElement via the ComboBox or via the mouse to set its properties. We achieve this by binding the ItemsSource of the ComboBox to the ScreenDesignPanel's Children collection, then binding their SelectedItems together. This works great.
However, for whatever reason, if the SelectedItem is a ContentControl or a subclass like Button the ItemTemplate specified for the ComboBox is ignored for the 'selected item area' but it is applied when displaying the item in the dropdown list. If the SelectedItem is not a ContentControl, the template is used in both cases.
This also is seemingly specific to the ComboBox. If we use any other selector control: ListBox, ListView, ItemsControl... even third-party ComboBox controls... they all work as expected, properly applying the DataTemplate. ComboBox is doing something internally which no other control is doing.
Note: Below is an over-simplified example for illustrative purposes of the issue only. It is not how we're actually using it as described above.
Also of note: In the DataTemplate for the ComboBox.ItemTemplate, we are only using properties (i.e. Foreground in the example), and are not displaying the DataContext (i.e. the actual ContentControl) itself. This is important because again, the actual control already exists on the ScreenDesignPanel and therefore can't be used for display in the ComboBox's ItemTemplate as it would have two parents which isn't allowed. In other words, it is being used purely as data here.
One last thing... we have a working solution in our app, which was to wrap the Children before binding it to the ComboBox.ItemsSource. However, I'm still curious as to why the ComboBox behaves the way it does which is SPECIFICALLY what I'm asking. (In other words, I'm not looking for other solutions to this design. We already have a working one. I'm looking for clarity on the odd behavior of the ComboBox itself.)
On to the code!
In the first example below, note how the data template is applied to everything in the dropdown, but the selected item area only uses a template if the selected item is not a ContentControl.
<ComboBox>
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="I am the template" Foreground="{Binding Foreground}" />
</DataTemplate>
</ComboBox.ItemTemplate>
<!-- Four 'Data' items for example only -->
<TextBlock Text="I am a Red TextBox" Foreground="Red"/>
<ListBox Foreground="Purple">
<ListBoxItem>I am a Purple ListBox</ListBoxItem>
</ListBox>
<ContentControl Content="I am a Blue ContentControl" Foreground="Blue" />
<Button Content="I am a Button with Green text" Foreground="Green" />
</ComboBox>
This second example shows that it is completely acceptable and fully supported to use a UIElement as the content of a ContentPresenter and still use a DataTemplate (via ContentTemplate) so you can use it in a purely-data role, allowing the template itself to define the visual appearance without displaying the UIElement itself, which is used purely as data here.
<ContentPresenter>
<ContentPresenter.ContentTemplate>
<DataTemplate>
<TextBlock Text="I am the ContentTemplate" Foreground="{Binding Foreground}" />
</DataTemplate>
</ContentPresenter.ContentTemplate>
<ContentPresenter.Content>
<Button Content="I am the button" Foreground="Green" />
</ContentPresenter.Content>
</ContentPresenter>
Again, the issue is specific to a ComboBox. I want to find out why the data template isn't applied in that single case, and how to force it to be applied, if possible.
Of note, ComboBox does define SelectionBoxItemTemplate which is separate from the regular ItemTemplate but the rub is that is read-only so you can't set it. We really don't want to re-template the ComboBox as that can mess up proper theming.

Have you tried explicitly setting the DataTemplate to the ContentControl.ContentTemplate property?:
<UserControl.Resources>
<DataTemplate x:Key="DataTemplate">
<TextBlock Text="{Binding Content,
StringFormat='Displayed via template: {0}'}" />
</DataTemplate>
</UserControl.Resources>
...
<ContentControl Content="ContentControl"
ContentTemplate="{StaticResource DataTemplate}" />

How should I bind a Combobox's ItemsSource to Content and Items

Problem
I have a UserControl which contains a ToggleButton and a ComboBox. The control will allow the user to choose a sort type (via ComboBox) and a Direction (via ToggleButton). I want to be able to expose some properties of the ComboBox and more, so how do I bind the ItemsSource of the ComboBox to an Items Property of the UserControl, which I will implement myself, but also the built-in Content property---similar to how a ComboBox can do both.
UserControl
I have a user control which set-up is similar as the below code, or look here.
<UserControl x:Class="Example.DirComboBox">
<Grid>
<ComboBox x:Name="cbItems" />
<ToggleButton x:Name="tbSortDir"/>
</Grid>
</UserControl>
Control Usage
I would like to be able to use it in two ways:
1:
Adding Child Elements.
<local:DirComboBox>
<ComboboxItem Content="Item 1"/>
</local:DirComboBox>
2:
Binding Items Property.
<local:DirComboBox Items="{Binding SortList}"/>
Alternatives
I would be willing to use alternatives, such as setting the root as ComboBox instead of UserControl but I need to expose the follow (but not sure how):
Have a ToggleButton on the side,
SortDirection property as a bool
RoutedEvent for Ascending and Descending

Define Dependancy Properties for SortDirection, Items in your usercontrol. Once you have these properties in your control you can directly set them from outside like:
<local:DirComboBox Items="{Binding SortList}" SortDirection="{Binding Sort}"/>
then inside your control bind these properties to respective controls like:
<UserControl x:Class="Example.DirComboBox">
<Grid>
<ComboBox x:Name="cbItems" ItemsSource="{Binding Items, RelativeSource={RelativeSource AncestorType={x:Type UserControl}}"}/>
<ToggleButton x:Name="tbSortDir" IsPressed="{Binding SortDirection, RelativeSource={RelativeSource AncestorType={x:Type UserControl}}"/>
</Grid>
</UserControl>
keep the binding mode as twoway.

Instead of basing my control on UserControl I changed to using a CustomControl as per the Tutorial on MSDN.

xaml / c# stackpanel with background rectangle

I have a list (ListBox) of items in XAML using a StackPanel- based element template. The layout is fine, but I would now like to have a rectangle as a background for each item - creating a box around each one.
I was thinking of using a Canvas somehow, but as each item's height varies (as well as the height of the items inside the StackPanel), I'm not sure how to do it (I'm new to C#/XAML). What would be the best composition for the template in this situation?

You can just specify it in an ItemTemplate and it will do what you want, something like;
<ListBox.ItemTemplate>
<DataTemplate>
<Grid>
<Border BorderBrush="Red" BorderThickness="2" Background="Blue"/>
<!-- Insert the rest of your Item template stuff here -->
</Grid>
</DataTemplate>
</ListBox.ItemTemplate>

ListBox is a type of ItemsControl, which exposes several properties to control the appearance of the items. In this case, have a look at ItemContainerStyle (in the case of ListBox, the item containers are instances of ListBoxItem). You could, for instance, set the Background property in an ItemsContainerStyle to some color.