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Fix altwin:swap_alt_win keymap option by querying XKB state #4521

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Fix altwin:swap_alt_win keymap option by querying XKB state #4521

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535f2cb
Initial plan
Copilot Dec 9, 2025
6a87c98
Fix altwin:swap_alt_win modifier tracking by querying XKB state
Copilot Dec 9, 2025
1e884e3
Address code review: Add documentation and optimize modifier loops
Copilot Dec 9, 2025
fc93528
Polish: Use find() instead of count() and add clarifying comment
Copilot Dec 9, 2025
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15 changes: 15 additions & 0 deletions include/common/mir/input/xkb_mapper.h
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Original file line number Diff line number Diff line change
Expand Up @@ -97,11 +97,26 @@ class XKBMapper : public KeyMapper
std::string& text) -> std::pair<xkb_keysym_t, bool>;
void press_modifier(MirInputEventModifiers mod);
void release_modifier(MirInputEventModifiers mod);
void update_modifier_from_xkb_state();

std::shared_ptr<Keymap> const keymap;
std::shared_ptr<xkb_keymap> const compiled_keymap;
XKBStatePtr state;
MirInputEventModifiers modifier_state{0};

// Tracks physically pressed keys (as XKB scan codes) to determine which physical keys
// are pressed when querying XKB state. Used to distinguish left/right modifier variants.
std::unordered_set<uint32_t> pressed_scancodes;

// Cached XKB modifier indices for performance. These are looked up once during
// construction from standard XKB modifier names and used to efficiently query
// XKB state for active modifiers.
xkb_mod_index_t mod_index_shift{XKB_MOD_INVALID};
xkb_mod_index_t mod_index_ctrl{XKB_MOD_INVALID};
xkb_mod_index_t mod_index_alt{XKB_MOD_INVALID};
xkb_mod_index_t mod_index_logo{XKB_MOD_INVALID};
xkb_mod_index_t mod_index_caps{XKB_MOD_INVALID};
xkb_mod_index_t mod_index_num{XKB_MOD_INVALID};
};

XkbMappingState* get_keymapping_state(MirInputDeviceId id);
Expand Down
113 changes: 99 additions & 14 deletions src/common/input/xkb_mapper.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -300,6 +300,13 @@ mircv::XKBMapper::XkbMappingState::XkbMappingState(
compiled_keymap{std::move(compiled_keymap)},
state{make_unique_state(this->compiled_keymap.get())}
{
// Cache XKB modifier indices for efficient lookup
mod_index_shift = xkb_keymap_mod_get_index(this->compiled_keymap.get(), XKB_MOD_NAME_SHIFT);
mod_index_ctrl = xkb_keymap_mod_get_index(this->compiled_keymap.get(), XKB_MOD_NAME_CTRL);
mod_index_alt = xkb_keymap_mod_get_index(this->compiled_keymap.get(), XKB_MOD_NAME_ALT);
mod_index_logo = xkb_keymap_mod_get_index(this->compiled_keymap.get(), XKB_MOD_NAME_LOGO);
mod_index_caps = xkb_keymap_mod_get_index(this->compiled_keymap.get(), XKB_MOD_NAME_CAPS);
mod_index_num = xkb_keymap_mod_get_index(this->compiled_keymap.get(), XKB_MOD_NAME_NUM);
}

void mircv::XKBMapper::XkbMappingState::set_key_state(std::vector<uint32_t> const& key_state)
Expand Down Expand Up @@ -365,18 +372,10 @@ auto mircv::XKBMapper::XkbMappingState::update_state(
-> std::pair<xkb_keysym_t, bool>
{
auto keysym = xkb_state_key_get_one_sym(state.get(), scan_code);
auto const mod_change = modifier_from_xkb_scan_code(scan_code);

// Occasionally, we see XKB_KEY_Meta_L where XKB_KEY_Alt_L is correct
if (mod_change == mir_input_event_modifier_alt_left)
{
keysym = XKB_KEY_Alt_L;
}

if(action == mir_keyboard_action_down || action == mir_keyboard_action_repeat)
{
char buffer[7];
// scan code? really? not keysym?
xkb_state_key_get_utf8(state.get(), scan_code, buffer, sizeof(buffer));
text = buffer;
}
Expand All @@ -388,16 +387,16 @@ auto mircv::XKBMapper::XkbMappingState::update_state(
if (action == mir_keyboard_action_up)
{
mask = xkb_state_update_key(state.get(), scan_code, XKB_KEY_UP);
// TODO get the modifier state from xkbcommon and apply it
// for all other modifiers manually track them here:
release_modifier(mod_change);
pressed_scancodes.erase(scan_code);
// Query XKB for the actual modifier state after key update
update_modifier_from_xkb_state();
}
else if (action == mir_keyboard_action_down)
{
mask = xkb_state_update_key(state.get(), scan_code, XKB_KEY_DOWN);
// TODO get the modifier state from xkbcommon and apply it
// for all other modifiers manually track them here:
press_modifier(mod_change);
pressed_scancodes.insert(scan_code);
// Query XKB for the actual modifier state after key update
update_modifier_from_xkb_state();
}

bool const xkb_modifiers_changed =
Expand Down Expand Up @@ -494,6 +493,92 @@ xkb_keysym_t mircv::XKBMapper::ComposeState::update_state(xkb_keysym_t mapped_ke
return mapped_key;
}

void mircv::XKBMapper::XkbMappingState::update_modifier_from_xkb_state()
{
// Query XKB for the actual modifier state instead of tracking manually by scan codes
// This ensures that XKB options like altwin:swap_alt_win are properly respected
MirInputEventModifiers new_modifier_state = mir_input_event_modifier_none;

// Check shift modifier and determine left/right based on which physical keys are pressed
if (mod_index_shift != XKB_MOD_INVALID &&
xkb_state_mod_index_is_active(state.get(), mod_index_shift, XKB_STATE_MODS_EFFECTIVE))
{
new_modifier_state |= mir_input_event_modifier_shift;
// Check which physical shift keys are pressed
if (pressed_scancodes.find(to_xkb_scan_code(KEY_LEFTSHIFT)) != pressed_scancodes.end())
new_modifier_state |= mir_input_event_modifier_shift_left;
if (pressed_scancodes.find(to_xkb_scan_code(KEY_RIGHTSHIFT)) != pressed_scancodes.end())
new_modifier_state |= mir_input_event_modifier_shift_right;
}

// Check ctrl modifier
if (mod_index_ctrl != XKB_MOD_INVALID &&
xkb_state_mod_index_is_active(state.get(), mod_index_ctrl, XKB_STATE_MODS_EFFECTIVE))
{
new_modifier_state |= mir_input_event_modifier_ctrl;
// Check which physical ctrl keys are pressed
if (pressed_scancodes.find(to_xkb_scan_code(KEY_LEFTCTRL)) != pressed_scancodes.end())
new_modifier_state |= mir_input_event_modifier_ctrl_left;
if (pressed_scancodes.find(to_xkb_scan_code(KEY_RIGHTCTRL)) != pressed_scancodes.end())
new_modifier_state |= mir_input_event_modifier_ctrl_right;
}

// Check alt and meta modifiers - these are where altwin:swap_alt_win affects behavior
// XKB tells us if Alt/Meta are active, we determine left/right based on keysyms of pressed keys
bool const alt_active = mod_index_alt != XKB_MOD_INVALID &&
xkb_state_mod_index_is_active(state.get(), mod_index_alt, XKB_STATE_MODS_EFFECTIVE);
bool const meta_active = mod_index_logo != XKB_MOD_INVALID &&
xkb_state_mod_index_is_active(state.get(), mod_index_logo, XKB_STATE_MODS_EFFECTIVE);

if (alt_active || meta_active)
{
if (alt_active)
new_modifier_state |= mir_input_event_modifier_alt;
if (meta_active)
new_modifier_state |= mir_input_event_modifier_meta;

// Single iteration to check all pressed keys for both Alt and Meta modifiers.
// We query the keysym of each pressed key because XKB options like altwin:swap_alt_win
// change which keysyms are produced (e.g., physical Win key produces Alt_L keysym).
// This allows us to correctly determine left/right variants after key remapping.
for (auto scan_code : pressed_scancodes)
{
auto keysym = xkb_state_key_get_one_sym(state.get(), scan_code);

if (alt_active)
{
if (keysym == XKB_KEY_Alt_L)
new_modifier_state |= mir_input_event_modifier_alt_left;
else if (keysym == XKB_KEY_Alt_R)
new_modifier_state |= mir_input_event_modifier_alt_right;
}

if (meta_active)
{
if (keysym == XKB_KEY_Super_L || keysym == XKB_KEY_Meta_L)
new_modifier_state |= mir_input_event_modifier_meta_left;
else if (keysym == XKB_KEY_Super_R || keysym == XKB_KEY_Meta_R)
new_modifier_state |= mir_input_event_modifier_meta_right;
}
}
}

// Check caps lock
if (mod_index_caps != XKB_MOD_INVALID &&
xkb_state_mod_index_is_active(state.get(), mod_index_caps, XKB_STATE_MODS_EFFECTIVE))
{
new_modifier_state |= mir_input_event_modifier_caps_lock;
}

// Check num lock
if (mod_index_num != XKB_MOD_INVALID &&
xkb_state_mod_index_is_active(state.get(), mod_index_num, XKB_STATE_MODS_EFFECTIVE))
{
new_modifier_state |= mir_input_event_modifier_num_lock;
}

modifier_state = new_modifier_state;
}

void mircv::XKBMapper::XkbMappingState::press_modifier(MirInputEventModifiers mod)
{
Expand Down