session.ui: Use minimum fold threshold policy for leaflet

This fixes https://gitlab.gnome.org/GNOME/fractal/-/issues/780
by using the sum of minimum sizes as the size to fold the
leaflet at.

data/resources/style.css

@@ -90,8 +90,7 @@ headerbar.flat {
 }
 
 .sidebar .room .highlight {
-  /* TODO: use correct color variable */
-  background-color: @theme_selected_bg_color;
+  background-color: @accent_bg_color;
 }
 
 /* Content */
@@ -147,7 +146,7 @@ headerbar.flat {
 }
 
 .message-entry > .view:focus {
-  border: 2px solid @theme_selected_bg_color;
+  border: 2px solid @accent_bg_color;
   padding: 5px;
 }
 

data/resources/ui/session.ui

@@ -46,6 +46,7 @@
             </child>
             <child>
               <object class="AdwLeaflet" id="content">
+                <property name="fold-threshold-policy">minimum</property>
                 <child>
                   <object class="Sidebar" id="sidebar">
                     <property name="compact" bind-source="content" bind-property="folded" bind-flags="sync-create"/>