Fixes #1151 by using the size excluding the header

Author: batzen

Changelog.md

@@ -8,6 +8,7 @@
   - [#1134](../../issues/1134) - Selected Tab on first render has a black border
   - [#1135](../../issues/1135) - Black line between Ribbon and Window (when using a colorful theme)
   - [#1125](../../issues/1125) - BackStage Back Button doesn't have an accessibility text.
+  - [#1151](../../issues/1151) - ComboBox Popup MinWidth too large
   - [#1152](../../issues/1152) - Using ObjectToImageConverter via StaticResource raises an exception (thanks @nishy2000)
 
 ## 10.0.0

Fluent.Ribbon/Themes/Controls/ComboBox.xaml

@@ -187,9 +187,9 @@
                     </ResourceDictionary>
                 </Popup.Resources>
                 <Fluent:ResizeableContentControl x:Name="PART_PopupContentControl"
-                                                 MinWidth="{TemplateBinding ActualWidth}"
-                                                 MinHeight="{TemplateBinding ActualHeight}"
-                                                 MaxHeight="{TemplateBinding MaxDropDownHeight}"
+                                                 MinWidth="{Binding ActualWidth, ElementName=PART_ContentBorder}"
+                                                 MinHeight="{Binding ActualHeight, ElementName=PART_ContentBorder}"
+                                                 MaxHeight="{TemplateBinding MaxDropDownHeight}" 
                                                  IsEnabled="{TemplateBinding IsEnabled}"
                                                  ResizeMode="{TemplateBinding ResizeMode}">
                     <Grid>