Set panel stretch defaults for setting editor. Update minimum list width.

packages/settingeditor/src/settingeditor.tsx

@@ -297,7 +297,12 @@ class SettingEditor extends Widget {
 
     // Allow the message queue (which includes fit requests that might disrupt
     // setting relative sizes) to clear before setting sizes.
-    requestAnimationFrame(() => { panel.setRelativeSizes(state.sizes); });
+    requestAnimationFrame(() => {
+      panel.setRelativeSizes(state.sizes);
+      SplitPanel.setStretch(this._list, 0);
+      SplitPanel.setStretch(this._instructions, 1);
+      SplitPanel.setStretch(this._editor, 1);
+    });
   }
 
   /**

packages/settingeditor/style/settingeditor.css

@@ -72,8 +72,8 @@
 
 
 #setting-editor .jp-PluginList {
-  min-width: 120px;
-  width: 120px;
+  min-width: 175px;
+  width: 175px;
 }