[Core] Add Chordal Hold, an "opposite hands rule" tap-hold option similar to Achordion, Bilateral Combinations.

data/mappings/info_config.hjson

@@ -200,6 +200,7 @@
     "SPLIT_WPM_ENABLE": {"info_key": "split.transport.sync.wpm", "value_type": "flag"},
 
     // Tapping
+    "CHORDAL_HOLD": {"info_key": "tapping.chordal_hold", "value_type": "flag"},
     "HOLD_ON_OTHER_KEY_PRESS": {"info_key": "tapping.hold_on_other_key_press", "value_type": "flag"},
     "HOLD_ON_OTHER_KEY_PRESS_PER_KEY": {"info_key": "tapping.hold_on_other_key_press_per_key", "value_type": "flag"},
     "PERMISSIVE_HOLD": {"info_key": "tapping.permissive_hold", "value_type": "flag"},

data/schemas/keyboard.jsonschema

@@ -422,7 +422,11 @@
                                 "h": {"$ref": "qmk.definitions.v1#/key_unit"},
                                 "w": {"$ref": "qmk.definitions.v1#/key_unit"},
                                 "x": {"$ref": "qmk.definitions.v1#/key_unit"},
-                                "y": {"$ref": "qmk.definitions.v1#/key_unit"}
+                                "y": {"$ref": "qmk.definitions.v1#/key_unit"},
+                                "hand": {
+                                    "type": "string",
+                                    "enum": ["L", "R", "*"]
+                                }
                             }
                         }
                     }
@@ -915,6 +919,7 @@
         "tapping": {
             "type": "object",
             "properties": {
+                "chordal_hold": {"type": "boolean"},
                 "force_hold": {"type": "boolean"},
                 "force_hold_per_key": {"type": "boolean"},
                 "ignore_mod_tap_interrupt": {"type": "boolean"},

docs/reference_info_json.md

@@ -74,6 +74,8 @@ You can create `info.json` files at every level under `qmk_firmware/keyboards/<k
         * The delay between keydown and keyup for tap events in milliseconds.
         * Default: `0` (no delay)
 * `tapping`
+    * `chordal_hold` <Badge type="info">Boolean</Badge>
+        * Default: `false`
     * `hold_on_other_key_press` <Badge type="info">Boolean</Badge>
         * Default: `false`
     * `hold_on_other_key_press_per_key` <Badge type="info">Boolean</Badge>
@@ -328,6 +330,8 @@ The ISO enter key is represented by a 1.25u×2uh key. Renderers which utilize in
                 * `h` <Badge type="info">KeyUnit</Badge>
                     * The height of the key, in key units.
                     * Default: `1` (1u)
+                * `hand` <Badge type="info">String</Badge>
+                    * The handedness of the key for Chordal Hold, either `"L"` (left hand), `"R"` (right hand), or `"*"` (either or exempted handedness).
                 * `label` <Badge type="info">String</Badge>
                     * What to name the key. This is *not* a key assignment as in the keymap, but should usually correspond to the keycode for the first layer of the default keymap.
                     * Example: `"Escape"`

docs/tap_hold.md

@@ -425,6 +425,248 @@ uint16_t get_quick_tap_term(uint16_t keycode, keyrecord_t *record) {
 If `QUICK_TAP_TERM` is set higher than `TAPPING_TERM`, it will default to `TAPPING_TERM`.
 :::
 
+## Chordal Hold
+
+Chordal Hold is intended to be used together with either Permissive Hold or Hold
+On Other Key Press. Chordal Hold is enabled by adding to your `config.h`:
+
+```c
+#define CHORDAL_HOLD
+```
+
+Chordal Hold implements, by default, an "opposite hands" rule. Suppose a
+tap-hold key is pressed and then, before the tapping term, another key is
+pressed. With Chordal Hold, the tap-hold key is settled as tapped if the two
+keys are on the same hand.
+
+Otherwise, if the keys are on opposite hands, Chordal Hold introduces no new
+behavior. Hold On Other Key Press or Permissive Hold may be used together with
+Chordal Hold to configure the behavior in the opposite hands case. With Hold On
+Other Key Press, an opposite hands chord is settled immediately as held. Or with
+Permissive Hold, an opposite hands chord is settled as held provided the other
+key is pressed and released (nested press) before releasing the tap-hold key.
+
+Chordal Hold may be useful to avoid accidental modifier activation with
+mod-taps, particularly in rolled keypresses when using home row mods.
+
+Notes:
+
+* Chordal Hold has no effect after the tapping term. 
+
+* Combos are exempt from the opposite hands rule, since "handedness" is
+  ill-defined in this case. Even so, Chordal Hold's behavior involving combos
+  may be customized through the `get_chordal_hold()` callback.
+
+An example of a sequence that is affected by “chordal hold”: 
+
+- `SFT_T(KC_A)` Down
+- `KC_C` Down
+- `KC_C` Up
+- `SFT_T(KC_A)` Up
+
+```
+                         TAPPING_TERM   
+  +---------------------------|--------+
+  | +----------------------+  |        |
+  | | SFT_T(KC_A)          |  |        |
+  | +----------------------+  |        |
+  |    +--------------+       |        |
+  |    | KC_C         |       |        |
+  |    +--------------+       |        |
+  +---------------------------|--------+
+```
+
+If the two keys are on the same hand, then this will produce `ac` with 
+`SFT_T(KC_A)` settled as tapped the moment that `KC_C` is pressed. 
+
+If the two keys are on opposite hands and the `HOLD_ON_OTHER_KEY_PRESS`
+option enabled, this will produce `C` with `SFT_T(KC_A)` settled as held the
+moment that `KC_C` is pressed.
+
+Or if the two keys are on opposite hands and the `PERMISSIVE_HOLD` option is
+enabled, this will produce `C` with `SFT_T(KC_A)` settled as held the
+moment that `KC_C` is released.
+
+### Chordal Hold Handedness
+
+Determining whether keys are on the same or opposite hands involves defining the
+"handedness" of each key position. By default, if nothing is specified,
+handedness is guessed based on keyboard geometry.
+
+Handedness may be specified with `chordal_hold_layout`. In keymap.c, define
+`chordal_hold_layout` in the following form:
+
+```c
+const char chordal_hold_layout[MATRIX_ROWS][MATRIX_COLS] PROGMEM =
+    LAYOUT(
+        'L', 'L', 'L', 'L', 'L', 'L',  'R', 'R', 'R', 'R', 'R', 'R', 
+        'L', 'L', 'L', 'L', 'L', 'L',  'R', 'R', 'R', 'R', 'R', 'R', 
+        'L', 'L', 'L', 'L', 'L', 'L',  'R', 'R', 'R', 'R', 'R', 'R', 
+                       'L', 'L', 'L',  'R', 'R', 'R'
+    );
+```
+
+Use the same `LAYOUT` macro as used to define your keymap layers. Each entry is
+a character indicating the handedness of one key, either `'L'` for left, `'R'`
+for right, or `'*'` to exempt keys from the "opposite hands rule." A key with
+`'*'` handedness may settle as held in chords with any other key. This could be
+used perhaps on thumb keys or other places where you want to allow same-hand
+chords.
+
+Keyboard makers may specify handedness in keyboard.json. Under `"layouts"`,
+specify the handedness of a key by adding a `"hand"` field with a value of
+either `"L"`, `"R"`, or `"*"`. Note that if `"layouts"` contains multiple
+layouts, only the first one is read. For example:
+
+```json
+{"matrix": [5, 6], "x": 0, "y": 5.5, "w": 1.25, "hand", "*"},
+```
+
+Alternatively, handedness may be defined functionally with
+`chordal_hold_handedness()`. For example, in keymap.c define:
+
+```c
+char chordal_hold_handedness(keypos_t key) {
+    if (key.col == 0 || key.col == MATRIX_COLS - 1) {
+        return '*';  // Exempt the outer columns.
+    }
+    // On split keyboards, typically, the first half of the rows are on the
+    // left, and the other half are on the right.
+    return key.row < MATRIX_ROWS / 2 ? 'L' : 'R';
+}
+```
+
+Given the matrix position of a key, the function should return `'L'`, `'R'`, or
+`'*'`. Adapt the logic in this function according to the keyboard's matrix.
+
+::: warning
+Note the matrix may have irregularities around larger keys, around the edges of
+the board, and around thumb clusters. You may find it helpful to use [this
+debugging example](faq_debug#which-matrix-position-is-this-keypress) to
+correspond physical keys to matrix positions.
+:::
+
+::: tip If you define both `chordal_hold_layout[MATRIX_ROWS][MATRIX_COLS]` and
+`chordal_hold_handedness(keypos_t key)` for handedness, the latter takes
+precedence.
+
+
+### Per-chord customization
+
+Beyond the per-key configuration possible through handedness, Chordal Hold may
+be configured at a *per-chord* granularity for detailed tuning. In keymap.c,
+define `get_chordal_hold()`. Returning true settles the chord as held, while
+returning false settles as tapped.
+
+For example:
+
+```c
+bool get_chordal_hold(uint16_t tap_hold_keycode, keyrecord_t* tap_hold_record,
+                      uint16_t other_keycode, keyrecord_t* other_record) {
+    // Exceptionally allow some one-handed chords for hotkeys.
+    switch (tap_hold_keycode) {
+        case LCTL_T(KC_Z):
+            if (other_keycode == KC_C || other_keycode == KC_V) {
+                return true;
+            }
+            break;
+
+        case RCTL_T(KC_SLSH):
+            if (other_keycode == KC_N) {
+                return true;
+            }
+            break;
+    }
+    // Otherwise defer to the opposite hands rule.
+    return get_chordal_hold_default(tap_hold_record, other_record);
+}
+```
+
+As shown in the last line above, you may use
+`get_chordal_hold_default(tap_hold_record, other_record)` to get the default tap
+vs. hold decision according to the opposite hands rule.
+
+If you use home row mods, you may want to produce a hotkey like Ctrl+Shift+V by
+holding Ctrl and Shift mod-taps on one hand while tapping `KC_V` with the other
+hand, say:
+
+- `RCTL_T(KC_K)` Down
+- `RSFT_T(KC_L)` Down (on the same hand as `RCTL_T(KC_K)`)
+- `KC_V` Down
+- `KC_V` Up
+- `RCTL_T(KC_K)` Up
+- `RSFT_T(KC_L)` Up
+
+However, supposing `RCTL_T(KC_K)` and `RSFT_T(KC_L)` are on the same hand,
+Chordal Hold by default considers `RCTL_T(KC_K)` tapped, producing "`kV`"
+instead of the desired Ctrl+Shift+V. 
+
+To address this, `get_chordal_hold()` may be defined to allow chords between any
+pair of mod-tap keys with
+
+```c
+bool get_chordal_hold(uint16_t tap_hold_keycode, keyrecord_t* tap_hold_record,
+                      uint16_t other_keycode, keyrecord_t* other_record) {
+    // Allow hold between any pair of mod-tap keys.
+    if (IS_QK_MOD_TAP(tap_hold_keycode) && IS_QK_MOD_TAP(other_keycode)) {
+        return true;
+    }
+
+    // Otherwise defer to the opposite hands rule.
+    return get_chordal_hold_default(tap_hold_record, other_record);
+}
+```
+
+Or to allow one-handed chords of specific mod-taps but not others, use:
+
+```c
+bool get_chordal_hold(uint16_t tap_hold_keycode, keyrecord_t* tap_hold_record,
+                      uint16_t other_keycode, keyrecord_t* other_record) {
+    switch (tap_hold_keycode) {
+        case RCTL_T(KC_K):  
+            if (other_keycode == RSFT_T(KC_L)) {
+                // Allow hold in "RCTL_T(KC_K) down, RSFT_T(KC_L) down".
+                return true;
+            }
+            break;
+
+        case RSFT_T(KC_L):
+            if (other_keycode == RCTL_T(KC_K)) {
+                // Allow hold in "RSFT_T(KC_L) down, RCTL_T(KC_K) down".
+                return true;
+            }
+            break;
+    }
+    // Otherwise defer to the opposite hands rule.
+    return get_chordal_hold_default(tap_hold_record, other_record);
+}
+```
+
+Above, two exceptions are defined, one where `RCTL_T(KC_K)` is pressed first and
+another where `RSFT_T(KC_L)` is held first, such that Ctrl+Shift+V could be done
+by holding the mod-taps in either order. For yet finer control, you could choose
+to define an exception for one order but not the other:
+
+```c
+bool get_chordal_hold(uint16_t tap_hold_keycode, keyrecord_t* tap_hold_record,
+                      uint16_t other_keycode, keyrecord_t* other_record) {
+    switch (tap_hold_keycode) {
+        case RCTL_T(KC_K):  
+            if (other_keycode == RSFT_T(KC_L)) {
+                // Allow hold in "RCTL_T(KC_K) down, RSFT_T(KC_L), down".
+                return true;
+            }
+            break;
+
+        // ... but RSFT_T(KC_L) is considered tapped in 
+        // "RSFT_T(KC_L) down, RCTL_T(KC_K) down".
+    }
+    // Otherwise defer to the opposite hands rule.
+    return get_chordal_hold_default(tap_hold_record, other_record);
+}
+```
+
+
 ## Retro Tapping
 
 To enable `retro tapping`, add the following to your `config.h`: