docs(conditional-layers): Document feature

docs/docs/features/conditional-layers.md

@@ -0,0 +1,56 @@
+---
+title: Conditional Layers
+---
+
+Conditional layers support activating a particular layer (called the `then-layer`) when all layers
+in a specified set (called the `if-layers`) are active. This feature generalizes what's commonly
+known as tri-layer support, allowing activation of two layers (usually called "lower" and "raise")
+to trigger a third (usually called "adjust").
+
+Another way to think of this feature is as a simple combo system for layers, just like the usual
+[combos for behaviors](combos.md).
+
+## Configuration
+
+Conditional layers are configured via a `conditional_layers` node in your `.keymap` file as follows:
+
+```
+/ {
+    conditional_layers {
+        compatible = "zmk,conditional-layers";
+        tri_layer {
+            if-layers = <1 2>;
+            then-layer = <3>;
+        };
+    };
+};
+```
+
+Each conditional layer configuration may have whatever name you like, but it's helpful to choose
+something self explanatory like `tri_layer`. The following properties are supported:
+
+- `if-layers` specifies a set of layer numbers, all of which must be active for the conditional
+  layer to trigger.
+- `then-layer` specifies a layer number that should be activated if and only if all the layers
+  specified in the `if-layers` property are active.
+
+Therefore, in this example, layer 3 ("adjust") will be activated if and only if both layers 1
+("lower") and 2 ("raise") are active.
+
+:::tip
+Since higher-numbered layers are processed first, a `then-layer` should generally have a higher
+number than its associated `if-layers` so the `then-layer` can be accessed when active.
+:::
+
+:::info
+Activating a `then-layer` in one conditional layer configuration can trigger the `if-layers`
+condition in another configuration, possibly repeatedly.
+:::
+
+:::caution
+When configured as a `then-layer`, a layer's activation status is entirely controlled by the
+conditional layers feature. Even if the layer is activated for another reason (such as a [momentary
+layer](../behaviors/layers.md#momentary-layer) behavior), it will be immediately deactivated if the
+associated `then-layers` configuration is not met. As such, we recommend avoiding using regular
+layer behaviors for `then-layer` targets.
+:::

docs/sidebars.js

@@ -11,6 +11,7 @@ module.exports = {
     Features: [
       "features/keymaps",
       "features/combos",
+      "features/conditional-layers",
       "features/debouncing",
       "features/displays",
       "features/encoders",