feat(behaviors): Add mod-morph `keep-mods`

* Update docs for mod-morph
* Add unit tests for mod-morph
* Add keep-mods to DT binding

Co-authored-by: Martin Aumüller <aumuell@reserv.at>
Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>

app/dts/bindings/behaviors/zmk,behavior-mod-morph.yaml

@@ -14,3 +14,6 @@ properties:
   mods:
     type: int
     required: true
+  keep-mods:
+    type: int
+    required: false

app/include/zmk/hid.h

@@ -135,6 +135,8 @@ int zmk_hid_register_mods(zmk_mod_flags_t explicit_modifiers);
 int zmk_hid_unregister_mods(zmk_mod_flags_t explicit_modifiers);
 int zmk_hid_implicit_modifiers_press(zmk_mod_flags_t implicit_modifiers);
 int zmk_hid_implicit_modifiers_release();
+int zmk_hid_masked_modifiers_set(zmk_mod_flags_t masked_modifiers);
+int zmk_hid_masked_modifiers_clear();
 
 int zmk_hid_keyboard_press(zmk_key_t key);
 int zmk_hid_keyboard_release(zmk_key_t key);

app/src/behaviors/behavior_mod_morph.c

@@ -27,6 +27,7 @@ struct behavior_mod_morph_config {
     struct zmk_behavior_binding normal_binding;
     struct zmk_behavior_binding morph_binding;
     zmk_mod_flags_t mods;
+    zmk_mod_flags_t masked_mods;
 };
 
 struct behavior_mod_morph_data {
@@ -45,6 +46,7 @@ static int on_mod_morph_binding_pressed(struct zmk_behavior_binding *binding,
     }
 
     if (zmk_hid_get_explicit_mods() & cfg->mods) {
+        zmk_hid_masked_modifiers_set(cfg->masked_mods);
         data->pressed_binding = (struct zmk_behavior_binding *)&cfg->morph_binding;
     } else {
         data->pressed_binding = (struct zmk_behavior_binding *)&cfg->normal_binding;
@@ -64,7 +66,10 @@ static int on_mod_morph_binding_released(struct zmk_behavior_binding *binding,
 
     struct zmk_behavior_binding *pressed_binding = data->pressed_binding;
     data->pressed_binding = NULL;
-    return behavior_keymap_binding_released(pressed_binding, event);
+    int err;
+    err = behavior_keymap_binding_released(pressed_binding, event);
+    zmk_hid_masked_modifiers_clear();
+    return err;
 }
 
 static const struct behavior_driver_api behavior_mod_morph_driver_api = {
@@ -88,6 +93,8 @@ static int behavior_mod_morph_init(const struct device *dev) { return 0; }
         .normal_binding = _TRANSFORM_ENTRY(0, n),                                                  \
         .morph_binding = _TRANSFORM_ENTRY(1, n),                                                   \
         .mods = DT_INST_PROP(n, mods),                                                             \
+        .masked_mods = COND_CODE_0(DT_INST_NODE_HAS_PROP(n, keep_mods), (DT_INST_PROP(n, mods)),   \
+                                   (DT_INST_PROP(n, mods) & ~DT_INST_PROP(n, keep_mods))),         \
     };                                                                                             \
     static struct behavior_mod_morph_data behavior_mod_morph_data_##n = {};                        \
     DEVICE_DT_INST_DEFINE(n, behavior_mod_morph_init, NULL, &behavior_mod_morph_data_##n,          \

app/src/hid.c

@@ -20,10 +20,12 @@ static struct zmk_hid_consumer_report consumer_report = {.report_id = 2, .body =
 // Only release the modifier if the count is 0.
 static int explicit_modifier_counts[8] = {0, 0, 0, 0, 0, 0, 0, 0};
 static zmk_mod_flags_t explicit_modifiers = 0;
+static zmk_mod_flags_t implicit_modifiers = 0;
+static zmk_mod_flags_t masked_modifiers = 0;
 
 #define SET_MODIFIERS(mods)                                                                        \
     {                                                                                              \
-        keyboard_report.body.modifiers = mods;                                                     \
+        keyboard_report.body.modifiers = (mods & ~masked_modifiers) | implicit_modifiers;          \
         LOG_DBG("Modifiers set to 0x%02X", keyboard_report.body.modifiers);                        \
     }
 
@@ -158,13 +160,29 @@ static inline int check_keyboard_usage(zmk_key_t usage) {
         }                                                                                          \
     }
 
-int zmk_hid_implicit_modifiers_press(zmk_mod_flags_t implicit_modifiers) {
+int zmk_hid_implicit_modifiers_press(zmk_mod_flags_t new_implicit_modifiers) {
+    implicit_modifiers = new_implicit_modifiers;
     zmk_mod_flags_t current = GET_MODIFIERS;
-    SET_MODIFIERS(explicit_modifiers | implicit_modifiers);
+    SET_MODIFIERS(explicit_modifiers);
     return current == GET_MODIFIERS ? 0 : 1;
 }
 
 int zmk_hid_implicit_modifiers_release() {
+    implicit_modifiers = 0;
+    zmk_mod_flags_t current = GET_MODIFIERS;
+    SET_MODIFIERS(explicit_modifiers);
+    return current == GET_MODIFIERS ? 0 : 1;
+}
+
+int zmk_hid_masked_modifiers_set(zmk_mod_flags_t new_masked_modifiers) {
+    masked_modifiers = new_masked_modifiers;
+    zmk_mod_flags_t current = GET_MODIFIERS;
+    SET_MODIFIERS(explicit_modifiers);
+    return current == GET_MODIFIERS ? 0 : 1;
+}
+
+int zmk_hid_masked_modifiers_clear() {
+    masked_modifiers = 0;
     zmk_mod_flags_t current = GET_MODIFIERS;
     SET_MODIFIERS(explicit_modifiers);
     return current == GET_MODIFIERS ? 0 : 1;

app/tests/mod-morph/1-no-morph/events.patterns

@@ -0,0 +1,8 @@
+s/.*hid_listener_keycode_pressed.*keycode/pressed: keycode/p
+s/.*hid_listener_keycode_released.*keycode/released: keycode/p
+s/.*hid_register_mod.*Modifiers set to /reg explicit: Modifiers set to /p
+s/.*hid_unregister_mod.*Modifiers set to /unreg explicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_press.*Modifiers set to /reg implicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_release.*Modifiers set to /unreg implicit: Modifiers set to /p
+s/.*hid_masked_modifiers_set.*Modifiers set to /mask mods: Modifiers set to /p
+s/.*hid_masked_modifiers_clear.*Modifiers set to /unmask mods: Modifiers set to /p

app/tests/mod-morph/1-no-morph/keycode_events.snapshot

@@ -0,0 +1,5 @@
+pressed: keycode 0x04 implicit_mods 0x00 explicit_mods 0x00
+reg implicit: Modifiers set to 0x00
+released: keycode 0x04 implicit_mods 0x00 explicit_mods 0x00
+unreg implicit: Modifiers set to 0x00
+unmask mods: Modifiers set to 0x00

app/tests/mod-morph/1-no-morph/native_posix_64.keymap

@@ -0,0 +1,11 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+#include "../behavior_keymap.dtsi"
+
+&kscan {
+	events = <
+		ZMK_MOCK_PRESS(0,1,10)
+		ZMK_MOCK_RELEASE(0,1,10)
+	>;
+};

app/tests/mod-morph/2a-masked-morph/events.patterns

@@ -0,0 +1,8 @@
+s/.*hid_listener_keycode_pressed.*keycode/pressed: keycode/p
+s/.*hid_listener_keycode_released.*keycode/released: keycode/p
+s/.*hid_register_mod.*Modifiers set to /reg explicit: Modifiers set to /p
+s/.*hid_unregister_mod.*Modifiers set to /unreg explicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_press.*Modifiers set to /reg implicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_release.*Modifiers set to /unreg implicit: Modifiers set to /p
+s/.*hid_masked_modifiers_set.*Modifiers set to /mask mods: Modifiers set to /p
+s/.*hid_masked_modifiers_clear.*Modifiers set to /unmask mods: Modifiers set to /p

app/tests/mod-morph/2a-masked-morph/keycode_events.snapshot

@@ -0,0 +1,12 @@
+pressed: keycode 0xE1 implicit_mods 0x00 explicit_mods 0x00
+reg explicit: Modifiers set to 0x02
+reg implicit: Modifiers set to 0x02
+mask mods: Modifiers set to 0x00
+pressed: keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+reg implicit: Modifiers set to 0x00
+released: keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+unreg implicit: Modifiers set to 0x00
+unmask mods: Modifiers set to 0x02
+released: keycode 0xE1 implicit_mods 0x00 explicit_mods 0x00
+unreg explicit: Modifiers set to 0x00
+unreg implicit: Modifiers set to 0x00

app/tests/mod-morph/2a-masked-morph/native_posix_64.keymap

@@ -0,0 +1,13 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+#include "../behavior_keymap.dtsi"
+
+&kscan {
+	events = <
+		ZMK_MOCK_PRESS(1,0,10)
+		ZMK_MOCK_PRESS(0,1,10)
+		ZMK_MOCK_RELEASE(0,1,10)
+		ZMK_MOCK_RELEASE(1,0,10)
+	>;
+};

app/tests/mod-morph/2b-masked-morph-implicit-overwrite/events.patterns

@@ -0,0 +1,8 @@
+s/.*hid_listener_keycode_pressed.*keycode/pressed: keycode/p
+s/.*hid_listener_keycode_released.*keycode/released: keycode/p
+s/.*hid_register_mod.*Modifiers set to /reg explicit: Modifiers set to /p
+s/.*hid_unregister_mod.*Modifiers set to /unreg explicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_press.*Modifiers set to /reg implicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_release.*Modifiers set to /unreg implicit: Modifiers set to /p
+s/.*hid_masked_modifiers_set.*Modifiers set to /mask mods: Modifiers set to /p
+s/.*hid_masked_modifiers_clear.*Modifiers set to /unmask mods: Modifiers set to /p

app/tests/mod-morph/2b-masked-morph-implicit-overwrite/keycode_events.snapshot

@@ -0,0 +1,12 @@
+pressed: keycode 0xE1 implicit_mods 0x00 explicit_mods 0x00
+reg explicit: Modifiers set to 0x02
+reg implicit: Modifiers set to 0x02
+mask mods: Modifiers set to 0x00
+pressed: keycode 0x05 implicit_mods 0x02 explicit_mods 0x00
+reg implicit: Modifiers set to 0x02
+released: keycode 0x05 implicit_mods 0x02 explicit_mods 0x00
+unreg implicit: Modifiers set to 0x00
+unmask mods: Modifiers set to 0x02
+released: keycode 0xE1 implicit_mods 0x00 explicit_mods 0x00
+unreg explicit: Modifiers set to 0x00
+unreg implicit: Modifiers set to 0x00

app/tests/mod-morph/2b-masked-morph-implicit-overwrite/native_posix_64.keymap

@@ -0,0 +1,37 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+
+/ {
+    behaviors {
+        mod_morph: mod_morph {
+            compatible = "zmk,behavior-mod-morph";
+            label = "MOD_MORPH_TEST";
+            #binding-cells = <0>;
+            bindings = <&kp A>, <&kp LS(B)>;  // implict mod overwrite
+            mods = <(MOD_LSFT|MOD_RSFT)>;
+        };
+    };
+
+	keymap {
+		compatible = "zmk,keymap";
+		label ="Default keymap";
+
+		default_layer {
+			bindings = <
+				&kp LEFT_ALT &mod_morph
+				&kp LEFT_SHIFT &kp RIGHT_SHIFT
+			>;
+		};
+	};
+};
+
+&kscan {
+	events = <
+		ZMK_MOCK_PRESS(1,0,10)
+		ZMK_MOCK_PRESS(0,1,10)
+		ZMK_MOCK_RELEASE(0,1,10)
+		ZMK_MOCK_RELEASE(1,0,10)
+	>;
+};
+

app/tests/mod-morph/2c-masked-morph-and-explicit-mods/events.patterns

@@ -0,0 +1,8 @@
+s/.*hid_listener_keycode_pressed.*keycode/pressed: keycode/p
+s/.*hid_listener_keycode_released.*keycode/released: keycode/p
+s/.*hid_register_mod.*Modifiers set to /reg explicit: Modifiers set to /p
+s/.*hid_unregister_mod.*Modifiers set to /unreg explicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_press.*Modifiers set to /reg implicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_release.*Modifiers set to /unreg implicit: Modifiers set to /p
+s/.*hid_masked_modifiers_set.*Modifiers set to /mask mods: Modifiers set to /p
+s/.*hid_masked_modifiers_clear.*Modifiers set to /unmask mods: Modifiers set to /p

app/tests/mod-morph/2c-masked-morph-and-explicit-mods/keycode_events.snapshot

@@ -0,0 +1,18 @@
+pressed: keycode 0xE2 implicit_mods 0x00 explicit_mods 0x00
+reg explicit: Modifiers set to 0x04
+reg implicit: Modifiers set to 0x04
+pressed: keycode 0xE1 implicit_mods 0x00 explicit_mods 0x00
+reg explicit: Modifiers set to 0x06
+reg implicit: Modifiers set to 0x06
+mask mods: Modifiers set to 0x04
+pressed: keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+reg implicit: Modifiers set to 0x04
+released: keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+unreg implicit: Modifiers set to 0x04
+unmask mods: Modifiers set to 0x06
+released: keycode 0xE1 implicit_mods 0x00 explicit_mods 0x00
+unreg explicit: Modifiers set to 0x04
+unreg implicit: Modifiers set to 0x04
+released: keycode 0xE2 implicit_mods 0x00 explicit_mods 0x00
+unreg explicit: Modifiers set to 0x00
+unreg implicit: Modifiers set to 0x00

app/tests/mod-morph/2c-masked-morph-and-explicit-mods/native_posix_64.keymap

@@ -0,0 +1,15 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+#include "../behavior_keymap.dtsi"
+
+&kscan {
+	events = <
+		ZMK_MOCK_PRESS(0,0,10)
+		ZMK_MOCK_PRESS(1,0,10)
+		ZMK_MOCK_PRESS(0,1,10)
+		ZMK_MOCK_RELEASE(0,1,10)
+		ZMK_MOCK_RELEASE(1,0,10)
+		ZMK_MOCK_RELEASE(0,0,10)
+	>;
+};

app/tests/mod-morph/2d-masked-morph-into-hold-tap-tap/events.patterns

@@ -0,0 +1,8 @@
+s/.*hid_listener_keycode_pressed.*keycode/pressed: keycode/p
+s/.*hid_listener_keycode_released.*keycode/released: keycode/p
+s/.*hid_register_mod.*Modifiers set to /reg explicit: Modifiers set to /p
+s/.*hid_unregister_mod.*Modifiers set to /unreg explicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_press.*Modifiers set to /reg implicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_release.*Modifiers set to /unreg implicit: Modifiers set to /p
+s/.*hid_masked_modifiers_set.*Modifiers set to /mask mods: Modifiers set to /p
+s/.*hid_masked_modifiers_clear.*Modifiers set to /unmask mods: Modifiers set to /p

app/tests/mod-morph/2d-masked-morph-into-hold-tap-tap/keycode_events.snapshot

@@ -0,0 +1,12 @@
+pressed: keycode 0xE1 implicit_mods 0x00 explicit_mods 0x00
+reg explicit: Modifiers set to 0x02
+reg implicit: Modifiers set to 0x02
+mask mods: Modifiers set to 0x00
+pressed: keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+reg implicit: Modifiers set to 0x00
+released: keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+unreg implicit: Modifiers set to 0x00
+unmask mods: Modifiers set to 0x02
+released: keycode 0xE1 implicit_mods 0x00 explicit_mods 0x00
+unreg explicit: Modifiers set to 0x00
+unreg implicit: Modifiers set to 0x00

app/tests/mod-morph/2d-masked-morph-into-hold-tap-tap/native_posix_64.keymap

@@ -0,0 +1,45 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+
+&kscan {
+	events = <
+        /* Shift + tap &mod_morph --> expect B (but get Shift + B) */
+		ZMK_MOCK_PRESS(0,0,10)
+		ZMK_MOCK_PRESS(0,1,10)
+		ZMK_MOCK_RELEASE(0,1,10)
+		ZMK_MOCK_RELEASE(0,0,10)
+	>;
+};
+
+/ {
+    behaviors {
+        mod_morph: mod_morph {
+            compatible = "zmk,behavior-mod-morph";
+            label = "MOD_MORPH_TEST";
+            #binding-cells = <0>;
+            bindings = <&kp A>, <&lt 1 B>;
+            mods = <(MOD_LSFT|MOD_RSFT)>;
+        };
+
+    };
+
+	keymap {
+		compatible = "zmk,keymap";
+		label ="Default keymap";
+
+		default_layer {
+			bindings = <
+				&kp LEFT_SHIFT &mod_morph
+				&kp C &none
+			>;
+		};
+
+        second_layer {
+			bindings = <
+				&trans &trans
+				&kp D &trans
+			>;
+		};
+	};
+};

app/tests/mod-morph/2e-masked-morph-into-hold-tap-hold/events.patterns

@@ -0,0 +1,8 @@
+s/.*hid_listener_keycode_pressed.*keycode/pressed: keycode/p
+s/.*hid_listener_keycode_released.*keycode/released: keycode/p
+s/.*hid_register_mod.*Modifiers set to /reg explicit: Modifiers set to /p
+s/.*hid_unregister_mod.*Modifiers set to /unreg explicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_press.*Modifiers set to /reg implicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_release.*Modifiers set to /unreg implicit: Modifiers set to /p
+s/.*hid_masked_modifiers_set.*Modifiers set to /mask mods: Modifiers set to /p
+s/.*hid_masked_modifiers_clear.*Modifiers set to /unmask mods: Modifiers set to /p

app/tests/mod-morph/2e-masked-morph-into-hold-tap-hold/keycode_events.snapshot

@@ -0,0 +1,12 @@
+pressed: keycode 0xE1 implicit_mods 0x00 explicit_mods 0x00
+reg explicit: Modifiers set to 0x02
+reg implicit: Modifiers set to 0x02
+mask mods: Modifiers set to 0x00
+pressed: keycode 0x07 implicit_mods 0x00 explicit_mods 0x00
+reg implicit: Modifiers set to 0x00
+released: keycode 0x07 implicit_mods 0x00 explicit_mods 0x00
+unreg implicit: Modifiers set to 0x00
+unmask mods: Modifiers set to 0x02
+released: keycode 0xE1 implicit_mods 0x00 explicit_mods 0x00
+unreg explicit: Modifiers set to 0x00
+unreg implicit: Modifiers set to 0x00

app/tests/mod-morph/2e-masked-morph-into-hold-tap-hold/native_posix_64.keymap

@@ -0,0 +1,47 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+
+&kscan {
+	events = <
+        /* Shift + hold &mod_morph --> expect and get D (no shift) */
+		ZMK_MOCK_PRESS(0,0,10)
+		ZMK_MOCK_PRESS(0,1,200)
+		ZMK_MOCK_PRESS(1,0,10)
+		ZMK_MOCK_RELEASE(1,0,10)
+		ZMK_MOCK_RELEASE(0,1,10)
+		ZMK_MOCK_RELEASE(0,0,10)
+	>;
+};
+
+/ {
+    behaviors {
+        mod_morph: mod_morph {
+            compatible = "zmk,behavior-mod-morph";
+            label = "MOD_MORPH_TEST";
+            #binding-cells = <0>;
+            bindings = <&kp A>, <&lt 1 B>;
+            mods = <(MOD_LSFT|MOD_RSFT)>;
+        };
+
+    };
+
+	keymap {
+		compatible = "zmk,keymap";
+		label ="Default keymap";
+
+		default_layer {
+			bindings = <
+				&kp LEFT_SHIFT &mod_morph
+				&kp C &none
+			>;
+		};
+
+        second_layer {
+			bindings = <
+				&trans &trans
+				&kp D &trans
+			>;
+		};
+	};
+};

app/tests/mod-morph/3-unmasked-morph/events.patterns

@@ -0,0 +1,8 @@
+s/.*hid_listener_keycode_pressed.*keycode/pressed: keycode/p
+s/.*hid_listener_keycode_released.*keycode/released: keycode/p
+s/.*hid_register_mod.*Modifiers set to /reg explicit: Modifiers set to /p
+s/.*hid_unregister_mod.*Modifiers set to /unreg explicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_press.*Modifiers set to /reg implicit: Modifiers set to /p
+s/.*hid_implicit_modifiers_release.*Modifiers set to /unreg implicit: Modifiers set to /p
+s/.*hid_masked_modifiers_set.*Modifiers set to /mask mods: Modifiers set to /p
+s/.*hid_masked_modifiers_clear.*Modifiers set to /unmask mods: Modifiers set to /p

app/tests/mod-morph/3-unmasked-morph/keycode_events.snapshot

@@ -0,0 +1,12 @@
+pressed: keycode 0xE1 implicit_mods 0x00 explicit_mods 0x00
+reg explicit: Modifiers set to 0x02
+reg implicit: Modifiers set to 0x02
+mask mods: Modifiers set to 0x02
+pressed: keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+reg implicit: Modifiers set to 0x02
+released: keycode 0x05 implicit_mods 0x00 explicit_mods 0x00
+unreg implicit: Modifiers set to 0x02
+unmask mods: Modifiers set to 0x02
+released: keycode 0xE1 implicit_mods 0x00 explicit_mods 0x00
+unreg explicit: Modifiers set to 0x00
+unreg implicit: Modifiers set to 0x00

app/tests/mod-morph/3-unmasked-morph/native_posix_64.keymap

@@ -0,0 +1,37 @@
+#include <dt-bindings/zmk/keys.h>
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/kscan_mock.h>
+
+/ {
+    behaviors {
+        mod_morph: mod_morph {
+            compatible = "zmk,behavior-mod-morph";
+            label = "MOD_MORPH_TEST";
+            #binding-cells = <0>;
+            bindings = <&kp A>, <&kp B>;
+            mods = <(MOD_LSFT|MOD_RSFT)>;
+            keep-mods = <(MOD_LSFT|MOD_RSFT)>;  // no masking
+        };
+    };
+
+	keymap {
+		compatible = "zmk,keymap";
+		label ="Default keymap";
+
+		default_layer {
+			bindings = <
+				&kp LEFT_ALT &mod_morph
+				&kp LEFT_SHIFT &kp RIGHT_SHIFT
+			>;
+		};
+	};
+};
+
+&kscan {
+	events = <
+		ZMK_MOCK_PRESS(1,0,10)
+		ZMK_MOCK_PRESS(0,1,10)
+		ZMK_MOCK_RELEASE(0,1,10)
+		ZMK_MOCK_RELEASE(1,0,10)
+	>;
+};

app/tests/mod-morph/behavior_keymap.dtsi

@@ -0,0 +1,23 @@
+/ {
+    behaviors {
+        mod_morph: mod_morph {
+            compatible = "zmk,behavior-mod-morph";
+            label = "MOD_MORPH_TEST";
+            #binding-cells = <0>;
+            bindings = <&kp A>, <&kp B>;
+            mods = <(MOD_LSFT|MOD_RSFT)>;
+        };
+    };
+
+	keymap {
+		compatible = "zmk,keymap";
+		label ="Default keymap";
+
+		default_layer {
+			bindings = <
+				&kp LEFT_ALT &mod_morph
+				&kp LEFT_SHIFT &kp RIGHT_SHIFT
+			>;
+		};
+	};
+};

app/tests/modifiers/mixed/kp-lctl-dn-mod-dn-lctl-up-mod-up/keycode_events.snapshot

@@ -7,7 +7,7 @@ mods: Modifiers set to 0x03
 released: usage_page 0x07 keycode 0xE0 implicit_mods 0x00 explicit_mods 0x00
 unreg: Modifier 0 count: 0
 unreg: Modifier 0 released
-unreg: Modifiers set to 0x00
+unreg: Modifiers set to 0x02
 mods: Modifiers set to 0x00
 released: usage_page 0x07 keycode 0x05 implicit_mods 0x02 explicit_mods 0x00
 mods: Modifiers set to 0x00

docs/docs/behaviors/mod-morph.md

@@ -14,8 +14,6 @@ The Mod-Morph behavior sends a different keypress, depending on whether a specif
 
 The Mod-Morph behavior acts as one of two keycodes, depending on if the required modifier is being held during the keypress.
 
-When the modifier is being held it is sent along with the morphed keycode. This can cause problems when the morphed keycode and modifier have an existing relationship (such as `shift-delete` or `ctrl-v` on many operating systems).
-
 ### Configuration
 
 An example of how to implement the mod-morph "Grave Escape":
@@ -31,10 +29,6 @@ An example of how to implement the mod-morph "Grave Escape":
             mods = <(MOD_LGUI|MOD_LSFT|MOD_RGUI|MOD_RSFT)>;
         };
     };
-
-    keymap {
-        ...
-    };
 };
 ```
 
@@ -71,3 +65,26 @@ Example:
 ```
 mods = <(MOD_LGUI|MOD_LSFT|MOD_RGUI|MOD_RSFT)>;
 ```
+
+### Advanced configuration
+
+`keep-mods`
+
+When a modifier specified in `mods` is being held, it won't be sent along with the morphed keycode unless it is also specified in `keep-mods`. By default `keep-mods` equals `0`, which means no modifier specified in `mods` will be sent along with the morphed keycode.
+
+For example, the following configuration morphs `LEFT_SHIFT` + `BACKSPACE` into `DELETE`, and morphs `RIGHT_SHIFT` + `BACKSPACE` into `RIGHT_SHIFT` + `DELETE`.
+
+```
+/ {
+    behaviors {
+        bspc_del: backspace_delete {
+            compatible = "zmk,behavior-mod-morph";
+            label = "BACKSPACE_DELETE";
+            #binding-cells = <0>;
+            bindings = <&kp BACKSPACE>, <&kp DELETE>;
+            mods = <(MOD_LSFT|MOD_RSFT)>;
+            keep-mods = <(MOD_RSFT)>;
+        };
+    };
+};
+```