add common issues page

astro.config.mjs

@@ -19,6 +19,7 @@ const config = [
     'Custom Service',
     'Subclassing GTK Widgets',
     'Examples',
+    'Common Issues'
 ];
 
 const widgets = [

src/content/docs/config/common-issues.md

@@ -0,0 +1,21 @@
+---
+title: Common Issues
+---
+
+## Window doesn't show up
+
+When a window doesn't have a size allocation on construction,
+it will never show up.
+You can solve this by wrapping the children in a box
+and forcing the box to have some size.
+For example when the window's direct child is a
+revealer that starts off with `reveal_child: false`
+
+```js
+Widget.Window({
+  child: Widget.Box({
+    css: 'padding: 1px;',
+    child: Widget.Revealer(),
+  }),
+})
+```