Behaviors: Add 'ignore-modifiers' option to sticky keys

To combine multiple sticky modifiers, the sticky keys must ignore
other (sticky) modifier keypresses.

This behavior is important for "callum-style mods", where all modifiers
are sticky mods.

Fixes #829

app/dts/behaviors/sticky_key.dtsi

@@ -12,6 +12,7 @@
 			#binding-cells = <1>;
 			release-after-ms = <1000>;
 			bindings = <&kp>;
+			ignore-modifiers;
 		};
 		/omit-if-no-ref/ sl: behavior_sticky_layer {
 			compatible = "zmk,behavior-sticky-key";

app/dts/bindings/behaviors/zmk,behavior-sticky-key.yaml

@@ -15,3 +15,5 @@ properties:
     type: int
   quick-release:
     type: boolean
+  ignore-modifiers:
+    type: boolean

app/src/behaviors/behavior_sticky_key.c

@@ -31,6 +31,7 @@ LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 struct behavior_sticky_key_config {
     uint32_t release_after_ms;
     bool quick_release;
+    bool ignore_modifiers;
     struct zmk_behavior_binding behavior;
 };
 
@@ -201,7 +202,7 @@ static int sticky_key_keycode_state_changed_listener(const zmk_event_t *eh) {
             continue;
         }
 
-        // If events were queued, the timer event may be queued late or not at all.
+        // If this event was queued, the timer may be triggered late or not at all.
         // Release the sticky key if the timer should've run out in the meantime.
         if (sticky_key->release_at != 0 && ev->timestamp > sticky_key->release_at) {
             stop_timer(sticky_key);
@@ -210,6 +211,11 @@ static int sticky_key_keycode_state_changed_listener(const zmk_event_t *eh) {
         }
 
         if (ev->state) { // key down
+            if (sticky_key->config->ignore_modifiers && is_mod(ev->usage_page, ev->keycode)) {
+                // ignore modifier key press so we can stack sticky keys and combine with other
+                // modifiers
+                continue;
+            }
             if (sticky_key->modified_key_usage_page != 0 || sticky_key->modified_key_keycode != 0) {
                 // this sticky key is already in use for a keycode
                 continue;
@@ -270,6 +276,7 @@ static struct behavior_sticky_key_data behavior_sticky_key_data;
     static struct behavior_sticky_key_config behavior_sticky_key_config_##n = {                    \
         .behavior = ZMK_KEYMAP_EXTRACT_BINDING(0, DT_DRV_INST(n)),                                 \
         .release_after_ms = DT_INST_PROP(n, release_after_ms),                                     \
+        .ignore_modifiers = DT_INST_PROP(n, ignore_modifiers),                                     \
         .quick_release = DT_INST_PROP(n, quick_release),                                           \
     };                                                                                             \
     DEVICE_DT_INST_DEFINE(n, behavior_sticky_key_init, device_pm_control_nop,                      \

app/tests/sticky-keys/10-callum-mods/events.patterns

@@ -0,0 +1 @@
+s/.*hid_listener_keycode_//p

app/tests/sticky-keys/10-callum-mods/keycode_events.snapshot

@@ -0,0 +1,8 @@
+pressed: usage_page 0x07 keycode 0xe0 implicit_mods 0x00 explicit_mods 0x00
+pressed: usage_page 0x07 keycode 0xe1 implicit_mods 0x00 explicit_mods 0x00
+pressed: usage_page 0x07 keycode 0x04 implicit_mods 0x00 explicit_mods 0x00
+released: usage_page 0x07 keycode 0xe0 implicit_mods 0x00 explicit_mods 0x00
+released: usage_page 0x07 keycode 0xe1 implicit_mods 0x00 explicit_mods 0x00
+released: usage_page 0x07 keycode 0x04 implicit_mods 0x00 explicit_mods 0x00
+pressed: usage_page 0x07 keycode 0x04 implicit_mods 0x00 explicit_mods 0x00
+released: usage_page 0x07 keycode 0x04 implicit_mods 0x00 explicit_mods 0x00

app/tests/sticky-keys/10-callum-mods/native_posix.keymap

@@ -0,0 +1,43 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+
+/ {
+	keymap {
+		compatible = "zmk,keymap";
+		label ="Default keymap";
+
+		default_layer {
+			bindings = <
+				&sk E &sl 1
+				&kp A &kp B>;
+		};
+
+		lower_layer {
+			bindings = <
+				&sk LEFT_CONTROL &kp X
+				&sk LEFT_SHIFT &kp Z>;
+		};
+	};
+};
+
+&kscan {
+	events = <
+		/* press sl lower_layer */
+		ZMK_MOCK_PRESS(0,1,10) 
+		/* tap sk LEFT_CONTROL */
+		ZMK_MOCK_PRESS(0,0,10)
+		ZMK_MOCK_RELEASE(0,0,10)
+		/* tap sk LEFT_SHIFT */
+		ZMK_MOCK_PRESS(1,0,10)
+		ZMK_MOCK_RELEASE(1,0,10)
+		/* release sl lower_layer */
+		ZMK_MOCK_RELEASE(0,1,10) 
+		/* tap A (with left control and left shift enabled) */
+		ZMK_MOCK_PRESS(1,0,10) 
+		ZMK_MOCK_RELEASE(1,0,10) 
+		/* tap A (no sticky keys anymore) */
+		ZMK_MOCK_PRESS(1,0,10) 
+		ZMK_MOCK_RELEASE(1,0,10) 
+	>;
+};