fx115: Scaffold: Fix Tests listbox sizing

- Remove splitter for now - no longer works with flex, so we'll need to
  rethink it
- Borrow selectItemsDialog richlistbox sizing fix via mixin

chrome/content/scaffold/scaffold.xhtml

@@ -502,7 +502,6 @@
 									</hbox>
 								</hbox>
 							</vbox>
-							<splitter/>
 							<iframe src="monaco/monaco.html" id="editor-tests" onmousedown="this.focus()"/>
 						</vbox>
 					</tabpanel>

scss/abstracts/_mixins.scss

@@ -307,4 +307,13 @@
 	--material-panedivider: 1px solid var(--color-panedivider);
 	--material-border-quinary: 1px solid var(--fill-quinary);
 	--material-border-quarternary: 1px solid var(--fill-quarternary);
-}
+}
+
+@mixin contain-richlistbox {
+	// richlistbox elements are crazy and will expand beyond the window size
+	// unless all/most elements in the hierarchy that contain that
+	// richlistbox have a min-height: 0 set
+	vbox, hbox {
+		min-height: 0;
+	}
+}

scss/components/_selectItemsDialog.scss

@@ -42,7 +42,5 @@
 // unless all/most elements in the hierarchy that contain that
 // richlistbox have a min-height: 0 set
 .contain-richlistbox {
-    vbox, hbox {
-        min-height: 0;
-    }
-}
+    @include contain-richlistbox;
+}

scss/scaffold.scss

@@ -31,6 +31,7 @@ $scaffold-toolbarbutton-icons: (
 
 #scaffold {
 	min-height: 600px;
+	@include contain-richlistbox;
 }
 
 .menubar-container {
@@ -178,4 +179,5 @@ vbox > splitter {
 
 #editor-tests {
 	flex: 2;
+	margin-top: 3px;
 }