wiki: Add a FAQ page

wiki/Configuration:-Layout.md

@@ -140,7 +140,7 @@ The difference is that the focus ring is drawn only around the active window, wh
 | ![](./img/focus-ring.png) | ![](./img/border.png) |
 
 > [!TIP]
-> By default focus ring and border are rendered as a solid background rectangle behind windows.
+> By default, focus ring and border are rendered as a solid background rectangle behind windows.
 > That is, they will show up through semitransparent windows.
 > This is because windows using client-side decorations can have an arbitrary shape.
 > 

wiki/FAQ.md

@@ -0,0 +1,15 @@
+### Why is the border/focus ring showing up through semitransparent windows?
+
+Uncomment the `prefer-no-csd` setting at the [top level](./Configuration:-Miscellaneous.md) of the config.
+Niri will draw focus rings and borders *around* windows that agree to omit their client-side decorations.
+
+By default, focus ring and border are rendered as a solid background rectangle behind windows.
+That is, they will show up through semitransparent windows.
+This is because windows using client-side decorations can have an arbitrary shape.
+
+### Why is the Waybar pop-up menu showing behind windows?
+
+Set `"layer": "top"` in your Waybar config.
+
+Niri currently draws pop-up menus on the same layer as their parent surface.
+By default, Waybar is on the `bottom` layer, which is behind windows, so Waybar pop-up menus also show behind windows.

wiki/_Sidebar.md

@@ -5,6 +5,7 @@
 * [`niri msg`](./IPC.md)
 * [VSCode, Chromium, WezTerm](./Application-Issues.md)
 * [Xwayland](./Xwayland.md)
+* [FAQ](./FAQ.md)
 
 ## Configuration
 * [Overview](./Configuration:-Overview.md)