add macos forward-to-ime-modifier-mask

- gate interpretKeyEvents: forwarding by a configurable modifier mask
- handle IME direct commits without preedit (e.g. SKK kana mode)
- swallow keys consumed via IMKTextInput.selectMode (SKK Ctrl+J, q)

Author: shiena

docs/docs/config.md

@@ -879,12 +879,20 @@ ignore-selection-foreground-color = false
   - Automatically updates position when cursor moves via keyboard, mouse, or any other method
   - Set to `false` to use system default IME positioning behavior
 
+- `forward-to-ime-modifier-mask` - Modifier mask deciding when a key event is forwarded to the macOS IME (default: `["shift", "ctrl", "alt", "super"]`)
+  - macOS only. Has no effect on other platforms.
+  - A key event is forwarded to the IME (`interpretKeyEvents:`) when no modifier is pressed, or when the pressed modifiers intersect this mask. Otherwise the event is handled directly by the application without going through the IME.
+  - Useful for input methods such as SKK that need to receive `Ctrl`+key combinations directly. Set to `["shift", "ctrl"]` to keep `Ctrl-J` (and similar conversion shortcuts) reaching the IME while still letting unmodified keys pass through.
+  - Accepted values (case-insensitive): `shift`, `ctrl` (alias `control`), `alt` (alias `option`), `super` (aliases `cmd`, `command`). Unknown values are ignored with a warning.
+  - The default keeps the historical behavior where every key event reaches the IME.
+
 Example:
 
 ```toml
 [keyboard]
 disable-ctlseqs-alt = false
 ime-cursor-positioning = true
+forward-to-ime-modifier-mask = ["shift", "ctrl"]
 ```
 
 ## line-height

docs/docs/features/ime-support.md

@@ -50,6 +50,45 @@ ime-cursor-positioning = true  # Enable IME cursor positioning (default)
 - `true` (default): IME popups appear at the cursor position
 - `false`: Use system default IME positioning behavior
 
+## Forwarding Modifier Keys to the IME (macOS)
+
+On macOS, Rio decides per-keypress whether to forward the event to the active
+input method (`interpretKeyEvents:`) or handle it directly. The
+`forward-to-ime-modifier-mask` option controls this decision.
+
+```toml
+[keyboard]
+# Default: every modifier combination is forwarded to the IME (historical behavior).
+forward-to-ime-modifier-mask = ["shift", "ctrl", "alt", "super"]
+```
+
+A key event is forwarded to the IME when no modifier is pressed, OR when the
+pressed modifiers intersect this mask. Otherwise the event is handled directly
+by Rio without going through the IME.
+
+### Use Case: SKK
+
+SKK and similar input methods rely on `Ctrl`+key combinations (e.g. `Ctrl-J` to
+toggle kana input) and `Shift`+letter for conversion. Keep these combinations
+reaching the IME while letting other modifiers (such as `Cmd`+key shortcuts)
+bypass it:
+
+```toml
+[keyboard]
+forward-to-ime-modifier-mask = ["shift", "ctrl"]
+```
+
+### Accepted Values
+
+Case-insensitive. Unknown entries are ignored with a warning.
+
+| Value | Modifier |
+|-------|----------|
+| `shift` | Shift |
+| `ctrl` (or `control`) | Control |
+| `alt` (or `option`) | Option / Alt |
+| `super` (or `cmd`, `command`) | Command |
+
 ## Benefits
 
 ### Enhanced User Experience

frontends/rioterm/src/bindings/mod.rs

@@ -731,7 +731,7 @@ pub fn default_key_bindings(config: &rio_backend::config::Config) -> Vec<KeyBind
     bindings.extend(platform_key_bindings(
         config.navigation.has_navigation_key_bindings(),
         config.navigation.use_split,
-        config.keyboard,
+        config.keyboard.clone(),
     ));
 
     // Add hint bindings

frontends/rioterm/src/router/window.rs

@@ -187,6 +187,7 @@ pub fn configure_window(winit_window: &Window, config: &Config) {
         // OnlyRight - The right `Option` key is treated as `Alt`.
         // Both - Both `Option` keys are treated as `Alt`.
         // None - No special handling is applied for `Option` key.
+        use rio_window::keyboard::ModifiersState;
         use rio_window::platform::macos::{OptionAsAlt, WindowExtMacOS};
 
         match config.option_as_alt.to_lowercase().as_str() {
@@ -195,6 +196,20 @@ pub fn configure_window(winit_window: &Window, config: &Config) {
             "right" => winit_window.set_option_as_alt(OptionAsAlt::OnlyRight),
             _ => {}
         }
+
+        let mut mask = ModifiersState::empty();
+        for name in &config.keyboard.forward_to_ime_modifier_mask {
+            match name.to_lowercase().as_str() {
+                "shift" => mask |= ModifiersState::SHIFT,
+                "ctrl" | "control" => mask |= ModifiersState::CONTROL,
+                "alt" | "option" => mask |= ModifiersState::ALT,
+                "super" | "cmd" | "command" => mask |= ModifiersState::SUPER,
+                other => tracing::warn!(
+                    "ignoring unknown forward-to-ime-modifier-mask value: {other:?}"
+                ),
+            }
+        }
+        winit_window.set_forward_to_ime_modifier_mask(mask);
     }
 
     let is_transparent = config.window.opacity < 1.;

frontends/rioterm/src/screen/mod.rs

@@ -252,7 +252,7 @@ impl Screen<'_> {
             split_active_color: config.colors.split_active,
             panel: config.panel,
             title: config.title.clone(),
-            keyboard: config.keyboard,
+            keyboard: config.keyboard.clone(),
             scrollback_history_limit: config.scrollback_history_limit,
         };
 
@@ -537,7 +537,7 @@ impl Screen<'_> {
             .set_multiplier_and_divider(config.scroll.multiplier, config.scroll.divider);
 
         // Update keyboard config in context manager
-        self.context_manager.config.keyboard = config.keyboard;
+        self.context_manager.config.keyboard = config.keyboard.clone();
 
         self.sugarloaf
             .set_background_color(Some(self.renderer.dynamic_background.1));

rio-backend/src/config/defaults.rs

@@ -148,6 +148,16 @@ pub fn default_ime_cursor_positioning() -> bool {
     true
 }
 
+#[inline]
+pub fn default_forward_to_ime_modifier_mask() -> Vec<String> {
+    vec![
+        String::from("shift"),
+        String::from("ctrl"),
+        String::from("alt"),
+        String::from("super"),
+    ]
+}
+
 pub fn default_config_file_content() -> String {
     String::from(
         "# See the full configuration reference: https://rioterm.com/docs/config\n",

rio-backend/src/config/keyboard.rs

@@ -1,8 +1,11 @@
 use serde::{Deserialize, Serialize};
 
-use super::defaults::{default_disable_ctlseqs_alt, default_ime_cursor_positioning};
+use super::defaults::{
+    default_disable_ctlseqs_alt, default_forward_to_ime_modifier_mask,
+    default_ime_cursor_positioning,
+};
 
-#[derive(Debug, Serialize, Deserialize, PartialEq, Clone, Copy)]
+#[derive(Debug, Serialize, Deserialize, PartialEq, Clone)]
 pub struct Keyboard {
     // Disable ctlseqs with ALT keys
     // For example: Terminal.app does not deal with ctlseqs with ALT keys
@@ -19,6 +22,20 @@ pub struct Keyboard {
         rename = "ime-cursor-positioning"
     )]
     pub ime_cursor_positioning: bool,
+
+    // Modifier mask deciding when a key event is forwarded to the macOS IME.
+    // A key event is forwarded when no modifier is pressed, or when the
+    // pressed modifiers intersect this mask. Otherwise the event is handled
+    // directly by the application without going through the IME.
+    //
+    // Accepted values (case-insensitive): "shift", "ctrl", "alt", "super".
+    // Useful for input methods like SKK that need to receive Ctrl+key
+    // combinations directly.
+    #[serde(
+        default = "default_forward_to_ime_modifier_mask",
+        rename = "forward-to-ime-modifier-mask"
+    )]
+    pub forward_to_ime_modifier_mask: Vec<String>,
 }
 
 #[allow(clippy::derivable_impls)]
@@ -30,6 +47,7 @@ impl Default for Keyboard {
             #[cfg(not(target_os = "macos"))]
             disable_ctlseqs_alt: false,
             ime_cursor_positioning: default_ime_cursor_positioning(),
+            forward_to_ime_modifier_mask: default_forward_to_ime_modifier_mask(),
         }
     }
 }

rio-window/src/platform/macos.rs

@@ -17,6 +17,7 @@
 use std::os::raw::c_void;
 
 use crate::event_loop::{ActiveEventLoop, EventLoopBuilder};
+use crate::keyboard::ModifiersState;
 use crate::monitor::MonitorHandle;
 use crate::window::{Window, WindowAttributes};
 
@@ -106,6 +107,21 @@ pub trait WindowExtMacOS {
     /// Getter for the [`WindowExtMacOS::set_option_as_alt`].
     fn option_as_alt(&self) -> OptionAsAlt;
 
+    /// Set which modifier keys cause a key event to be forwarded to the macOS
+    /// IME (`interpretKeyEvents:`).
+    ///
+    /// A key event is forwarded to the IME when no modifier is pressed, OR
+    /// when the pressed modifiers intersect this mask. Otherwise the event is
+    /// handled directly by the application without going through the IME.
+    ///
+    /// This is useful for input methods (e.g. SKK) that need to receive
+    /// `Ctrl`/`Shift` combinations directly instead of having them processed
+    /// as terminal control sequences.
+    fn set_forward_to_ime_modifier_mask(&self, mask: ModifiersState);
+
+    /// Getter for the [`WindowExtMacOS::set_forward_to_ime_modifier_mask`].
+    fn forward_to_ime_modifier_mask(&self) -> ModifiersState;
+
     /// Makes the titlebar bigger, effectively adding more space around the
     /// window controls if the titlebar is invisible.
     fn set_unified_titlebar(&self, unified_titlebar: bool);
@@ -204,6 +220,18 @@ impl WindowExtMacOS for Window {
         self.window.maybe_wait_on_main(|w| w.option_as_alt())
     }
 
+    #[inline]
+    fn set_forward_to_ime_modifier_mask(&self, mask: ModifiersState) {
+        self.window
+            .maybe_queue_on_main(move |w| w.set_forward_to_ime_modifier_mask(mask))
+    }
+
+    #[inline]
+    fn forward_to_ime_modifier_mask(&self) -> ModifiersState {
+        self.window
+            .maybe_wait_on_main(|w| w.forward_to_ime_modifier_mask())
+    }
+
     #[inline]
     fn set_unified_titlebar(&self, unified_titlebar: bool) {
         self.window