[Core] Caps Word "Invert on shift" option: pressing Shift inverts the shift state.

data/mappings/info_config.hjson

@@ -28,6 +28,7 @@
     "BOOTMAGIC_LITE_COLUMN_RIGHT": {"info_key": "split.bootmagic.matrix.1", "value_type": "int"},
     "BOTH_SHIFTS_TURNS_ON_CAPS_WORD": {"info_key": "caps_word.both_shifts_turns_on", "value_type": "bool"},
     "CAPS_WORD_IDLE_TIMEOUT": {"info_key": "caps_word.idle_timeout", "value_type": "int"},
+    "CAPS_WORD_INVERT_ON_SHIFT": {"info_key": "caps_word.invert_on_shift", "value_type": "bool"},
     "COMBO_COUNT": {"info_key": "combo.count", "value_type": "int"},
     "COMBO_TERM": {"info_key": "combo.term", "value_type": "int"},
     "DEBOUNCE": {"info_key": "debounce", "value_type": "int"},

data/schemas/keyboard.jsonschema

@@ -227,7 +227,8 @@
                 "enabled": {"type": "boolean"},
                 "both_shifts_turns_on": {"type": "boolean"},
                 "double_tap_shift_turns_on": {"type": "boolean"},
-                "idle_timeout": {"$ref": "qmk.definitions.v1#/unsigned_int"}
+                "idle_timeout": {"$ref": "qmk.definitions.v1#/unsigned_int"},
+                "invert_on_shift": {"type": "boolean"}
             }
         },
         "combo": {

docs/feature_caps_word.md

@@ -90,6 +90,26 @@ by defining `IS_COMMAND()` in config.h:
 
 ## Customizing Caps Word :id=customizing-caps-word
 
+### Invert on shift :id=invert-on-shift
+
+By default, Caps Word turns off when Shift keys are pressed, considering them as
+word-breaking. Alternatively with the `CAPS_WORD_INVERT_ON_SHIFT` option,
+pressing the Shift key continues Caps Word and inverts the shift state. This
+is convenient for uncapitalizing one or a few letters within a word, for
+example with Caps Word on, typing "D, B, Shift+A, Shift+A, S" produces "DBaaS",
+or typing "P, D, F, Shift+S" produces "PDFs".
+
+Enable it by adding in config.h
+
+```c
+#define CAPS_WORD_INVERT_ON_SHIFT
+```
+
+This option works with regular Shift keys `KC_LSFT` and `KC_RSFT`, mod-tap Shift
+keys, and one-shot Shift keys. Note that while Caps Word is on, one-shot Shift
+keys behave like regular Shift keys, and have effect only while they are held.
+
+
 ### Idle timeout :id=idle-timeout
 
 Caps Word turns off automatically if no keys are pressed for

quantum/process_keycode/process_caps_word.c

@@ -14,13 +14,66 @@
 
 #include "process_caps_word.h"
 
+#ifdef CAPS_WORD_INVERT_ON_SHIFT
+static uint8_t held_mods = 0;
+
+static bool handle_shift(uint16_t keycode, keyrecord_t* record) {
+    switch (keycode) {
+        case OSM(MOD_LSFT):
+            keycode = KC_LSFT;
+            break;
+        case OSM(MOD_RSFT):
+            keycode = KC_RSFT;
+            break;
+
+#    ifndef NO_ACTION_TAPPING
+        case QK_MOD_TAP ... QK_MOD_TAP_MAX:
+            if (record->tap.count == 0) { // Mod-tap key is held.
+                switch (QK_MOD_TAP_GET_MODS(keycode)) {
+                    case MOD_LSFT:
+                        keycode = KC_LSFT;
+                        break;
+                    case MOD_RSFT:
+                        keycode = KC_RSFT;
+                        break;
+                }
+            }
+#    endif // NO_ACTION_TAPPING
+    }
+
+    if (keycode == KC_LSFT || keycode == KC_RSFT) {
+        const uint8_t mod = MOD_BIT(keycode);
+
+        if (is_caps_word_on()) {
+            if (record->event.pressed) {
+                held_mods |= mod;
+            } else {
+                held_mods &= ~mod;
+            }
+            return false;
+        } else if ((held_mods & mod) != 0) {
+            held_mods &= ~mod;
+            del_mods(mod);
+            return record->event.pressed;
+        }
+    }
+
+    return true;
+}
+#endif // CAPS_WORD_INVERT_ON_SHIFT
+
 bool process_caps_word(uint16_t keycode, keyrecord_t* record) {
     if (keycode == QK_CAPS_WORD_TOGGLE) {
         if (record->event.pressed) {
             caps_word_toggle();
         }
         return false;
     }
+#ifdef CAPS_WORD_INVERT_ON_SHIFT
+    if (!handle_shift(keycode, record)) {
+        return false;
+    }
+#endif // CAPS_WORD_INVERT_ON_SHIFT
 
 #ifndef NO_ACTION_ONESHOT
     const uint8_t mods = get_mods() | get_oneshot_mods();
@@ -111,19 +164,24 @@ bool process_caps_word(uint16_t keycode, keyrecord_t* record) {
                 if (record->tap.count == 0) { // Mod-tap key is held.
                     const uint8_t mods = QK_MOD_TAP_GET_MODS(keycode);
                     switch (mods) {
+#    ifndef CAPS_WORD_INVERT_ON_SHIFT
                         case MOD_LSFT:
                             keycode = KC_LSFT;
                             break;
                         case MOD_RSFT:
                             keycode = KC_RSFT;
                             break;
+#    endif // CAPS_WORD_INVERT_ON_SHIFT
                         case MOD_RSFT | MOD_RALT:
                             keycode = RSFT(KC_RALT);
                             break;
                         case MOD_RALT:
                             return true;
                         default:
                             caps_word_off();
+#    ifdef CAPS_WORD_INVERT_ON_SHIFT
+                            add_mods(held_mods);
+#    endif // CAPS_WORD_INVERT_ON_SHIFT
                             return true;
                     }
                 } else {
@@ -163,12 +221,20 @@ bool process_caps_word(uint16_t keycode, keyrecord_t* record) {
         clear_weak_mods();
 #endif // AUTO_SHIFT_ENABLE
         if (caps_word_press_user(keycode)) {
+#ifdef CAPS_WORD_INVERT_ON_SHIFT
+            if (held_mods) {
+                set_weak_mods(get_weak_mods() ^ MOD_BIT(KC_LSFT));
+            }
+#endif // CAPS_WORD_INVERT_ON_SHIFT
             send_keyboard_report();
             return true;
         }
     }
 
     caps_word_off();
+#ifdef CAPS_WORD_INVERT_ON_SHIFT
+    add_mods(held_mods);
+#endif // CAPS_WORD_INVERT_ON_SHIFT
     return true;
 }
 

tests/caps_word/caps_word_invert_on_shift/config.h

@@ -0,0 +1,21 @@
+// Copyright 2023 Google LLC
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 2 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+#pragma once
+
+#include "test_common.h"
+
+#define CAPS_WORD_INVERT_ON_SHIFT
+#define PERMISSIVE_HOLD

tests/caps_word/caps_word_invert_on_shift/test.mk

@@ -0,0 +1,17 @@
+# Copyright 2023 Google LLC
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+CAPS_WORD_ENABLE = yes
+