vt: add mouse encoding C API

Expose the internal mouse encoding functionality through the C API,
following the same pattern as the existing key encoding API. This
allows external consumers of libvt to encode mouse events into
terminal escape sequences (X10, UTF-8, SGR, URxvt, SGR-Pixels).

The API is split into two opaque handle types: GhosttyMouseEvent
for building normalized mouse events (action, button, modifiers,
position) and GhosttyMouseEncoder for converting those events into
escape sequences. The encoder is configured via a setopt interface
supporting tracking mode, output format, renderer geometry, button
state, and optional motion deduplication by last cell.

Encoder state can also be bulk-configured from a terminal handle
via ghostty_mouse_encoder_setopt_from_terminal. Failed encodes due
to insufficient buffer space report the required size without
mutating deduplication state.

include/ghostty/vt.h

@@ -31,6 +31,7 @@
  * - @ref terminal "Terminal" - Complete terminal emulator state and rendering
  * - @ref formatter "Formatter" - Format terminal content as plain text, VT sequences, or HTML
  * - @ref key "Key Encoding" - Encode key events into terminal sequences
+ * - @ref mouse "Mouse Encoding" - Encode mouse events into terminal sequences
  * - @ref osc "OSC Parser" - Parse OSC (Operating System Command) sequences
  * - @ref sgr "SGR Parser" - Parse SGR (Select Graphic Rendition) sequences
  * - @ref paste "Paste Utilities" - Validate paste data safety
@@ -88,6 +89,7 @@ extern "C" {
 #include <ghostty/vt/osc.h>
 #include <ghostty/vt/sgr.h>
 #include <ghostty/vt/key.h>
+#include <ghostty/vt/mouse.h>
 #include <ghostty/vt/paste.h>
 #include <ghostty/vt/wasm.h>
 

include/ghostty/vt/mouse.h

@@ -0,0 +1,35 @@
+/**
+ * @file mouse.h
+ *
+ * Mouse encoding module - encode mouse events into terminal escape sequences.
+ */
+
+#ifndef GHOSTTY_VT_MOUSE_H
+#define GHOSTTY_VT_MOUSE_H
+
+/** @defgroup mouse Mouse Encoding
+ *
+ * Utilities for encoding mouse events into terminal escape sequences,
+ * supporting X10, UTF-8, SGR, URxvt, and SGR-Pixels mouse protocols.
+ *
+ * ## Basic Usage
+ *
+ * 1. Create an encoder instance with ghostty_mouse_encoder_new().
+ * 2. Configure encoder options with ghostty_mouse_encoder_setopt() or
+ *    ghostty_mouse_encoder_setopt_from_terminal().
+ * 3. For each mouse event:
+ *    - Create a mouse event with ghostty_mouse_event_new().
+ *    - Set event properties (action, button, modifiers, position).
+ *    - Encode with ghostty_mouse_encoder_encode().
+ *    - Free the event with ghostty_mouse_event_free() or reuse it.
+ * 4. Free the encoder with ghostty_mouse_encoder_free() when done.
+ *
+ * @{
+ */
+
+#include <ghostty/vt/mouse/event.h>
+#include <ghostty/vt/mouse/encoder.h>
+
+/** @} */
+
+#endif /* GHOSTTY_VT_MOUSE_H */

include/ghostty/vt/mouse/encoder.h

@@ -0,0 +1,211 @@
+/**
+ * @file encoder.h
+ *
+ * Mouse event encoding to terminal escape sequences.
+ */
+
+#ifndef GHOSTTY_VT_MOUSE_ENCODER_H
+#define GHOSTTY_VT_MOUSE_ENCODER_H
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <ghostty/vt/allocator.h>
+#include <ghostty/vt/mouse/event.h>
+#include <ghostty/vt/terminal.h>
+#include <ghostty/vt/types.h>
+
+/**
+ * Opaque handle to a mouse encoder instance.
+ *
+ * This handle represents a mouse encoder that converts normalized
+ * mouse events into terminal escape sequences.
+ *
+ * @ingroup mouse
+ */
+typedef struct GhosttyMouseEncoder *GhosttyMouseEncoder;
+
+/**
+ * Mouse tracking mode.
+ *
+ * @ingroup mouse
+ */
+typedef enum {
+  /** Mouse reporting disabled. */
+  GHOSTTY_MOUSE_TRACKING_NONE = 0,
+
+  /** X10 mouse mode. */
+  GHOSTTY_MOUSE_TRACKING_X10 = 1,
+
+  /** Normal mouse mode (button press/release only). */
+  GHOSTTY_MOUSE_TRACKING_NORMAL = 2,
+
+  /** Button-event tracking mode. */
+  GHOSTTY_MOUSE_TRACKING_BUTTON = 3,
+
+  /** Any-event tracking mode. */
+  GHOSTTY_MOUSE_TRACKING_ANY = 4,
+} GhosttyMouseTrackingMode;
+
+/**
+ * Mouse output format.
+ *
+ * @ingroup mouse
+ */
+typedef enum {
+  GHOSTTY_MOUSE_FORMAT_X10 = 0,
+  GHOSTTY_MOUSE_FORMAT_UTF8 = 1,
+  GHOSTTY_MOUSE_FORMAT_SGR = 2,
+  GHOSTTY_MOUSE_FORMAT_URXVT = 3,
+  GHOSTTY_MOUSE_FORMAT_SGR_PIXELS = 4,
+} GhosttyMouseFormat;
+
+/**
+ * Mouse encoder size and geometry context.
+ *
+ * This describes the rendered terminal geometry used to convert
+ * surface-space positions into encoded coordinates.
+ *
+ * @ingroup mouse
+ */
+typedef struct {
+  /** Size of this struct in bytes. Must be set to sizeof(GhosttyMouseEncoderSize). */
+  size_t size;
+
+  /** Full screen width in pixels. */
+  uint32_t screen_width;
+
+  /** Full screen height in pixels. */
+  uint32_t screen_height;
+
+  /** Cell width in pixels. Must be non-zero. */
+  uint32_t cell_width;
+
+  /** Cell height in pixels. Must be non-zero. */
+  uint32_t cell_height;
+
+  /** Top padding in pixels. */
+  uint32_t padding_top;
+
+  /** Bottom padding in pixels. */
+  uint32_t padding_bottom;
+
+  /** Right padding in pixels. */
+  uint32_t padding_right;
+
+  /** Left padding in pixels. */
+  uint32_t padding_left;
+} GhosttyMouseEncoderSize;
+
+/**
+ * Mouse encoder option identifiers.
+ *
+ * These values are used with ghostty_mouse_encoder_setopt() to configure
+ * the behavior of the mouse encoder.
+ *
+ * @ingroup mouse
+ */
+typedef enum {
+  /** Mouse tracking mode (value: GhosttyMouseTrackingMode). */
+  GHOSTTY_MOUSE_ENCODER_OPT_EVENT = 0,
+
+  /** Mouse output format (value: GhosttyMouseFormat). */
+  GHOSTTY_MOUSE_ENCODER_OPT_FORMAT = 1,
+
+  /** Renderer size context (value: GhosttyMouseEncoderSize). */
+  GHOSTTY_MOUSE_ENCODER_OPT_SIZE = 2,
+
+  /** Whether any mouse button is currently pressed (value: bool). */
+  GHOSTTY_MOUSE_ENCODER_OPT_ANY_BUTTON_PRESSED = 3,
+
+  /** Whether to enable motion deduplication by last cell (value: bool). */
+  GHOSTTY_MOUSE_ENCODER_OPT_TRACK_LAST_CELL = 4,
+} GhosttyMouseEncoderOption;
+
+/**
+ * Create a new mouse encoder instance.
+ *
+ * @param allocator Pointer to allocator, or NULL to use the default allocator
+ * @param encoder Pointer to store the created encoder handle
+ * @return GHOSTTY_SUCCESS on success, or an error code on failure
+ *
+ * @ingroup mouse
+ */
+GhosttyResult ghostty_mouse_encoder_new(const GhosttyAllocator *allocator,
+                                        GhosttyMouseEncoder *encoder);
+
+/**
+ * Free a mouse encoder instance.
+ *
+ * @param encoder The encoder handle to free (may be NULL)
+ *
+ * @ingroup mouse
+ */
+void ghostty_mouse_encoder_free(GhosttyMouseEncoder encoder);
+
+/**
+ * Set an option on the mouse encoder.
+ *
+ * A null pointer value does nothing. It does not reset to defaults.
+ *
+ * @param encoder The encoder handle, must not be NULL
+ * @param option The option to set
+ * @param value Pointer to option value (type depends on option)
+ *
+ * @ingroup mouse
+ */
+void ghostty_mouse_encoder_setopt(GhosttyMouseEncoder encoder,
+                                  GhosttyMouseEncoderOption option,
+                                  const void *value);
+
+/**
+ * Set encoder options from a terminal's current state.
+ *
+ * This sets tracking mode and output format from terminal state.
+ * It does not modify size or any-button state.
+ *
+ * @param encoder The encoder handle, must not be NULL
+ * @param terminal The terminal handle, must not be NULL
+ *
+ * @ingroup mouse
+ */
+void ghostty_mouse_encoder_setopt_from_terminal(GhosttyMouseEncoder encoder,
+                                                GhosttyTerminal terminal);
+
+/**
+ * Reset internal encoder state.
+ *
+ * This clears motion deduplication state (last tracked cell).
+ *
+ * @param encoder The encoder handle (may be NULL)
+ *
+ * @ingroup mouse
+ */
+void ghostty_mouse_encoder_reset(GhosttyMouseEncoder encoder);
+
+/**
+ * Encode a mouse event into a terminal escape sequence.
+ *
+ * Not all mouse events produce output. In such cases this returns
+ * GHOSTTY_SUCCESS with out_len set to 0.
+ *
+ * If the output buffer is too small, this returns GHOSTTY_OUT_OF_MEMORY
+ * and out_len contains the required size.
+ *
+ * @param encoder The encoder handle, must not be NULL
+ * @param event The mouse event to encode, must not be NULL
+ * @param out_buf Buffer to write encoded bytes to, or NULL to query required size
+ * @param out_buf_size Size of out_buf in bytes
+ * @param out_len Pointer to store bytes written (or required bytes on failure)
+ * @return GHOSTTY_SUCCESS on success, GHOSTTY_OUT_OF_MEMORY if buffer is too small,
+ *         or another error code
+ *
+ * @ingroup mouse
+ */
+GhosttyResult ghostty_mouse_encoder_encode(GhosttyMouseEncoder encoder,
+                                           GhosttyMouseEvent event,
+                                           char *out_buf,
+                                           size_t out_buf_size,
+                                           size_t *out_len);
+
+#endif /* GHOSTTY_VT_MOUSE_ENCODER_H */

include/ghostty/vt/mouse/event.h

@@ -0,0 +1,193 @@
+/**
+ * @file event.h
+ *
+ * Mouse event representation and manipulation.
+ */
+
+#ifndef GHOSTTY_VT_MOUSE_EVENT_H
+#define GHOSTTY_VT_MOUSE_EVENT_H
+
+#include <stdbool.h>
+#include <ghostty/vt/allocator.h>
+#include <ghostty/vt/key/event.h>
+#include <ghostty/vt/types.h>
+
+/**
+ * Opaque handle to a mouse event.
+ *
+ * This handle represents a normalized mouse input event containing
+ * action, button, modifiers, and surface-space position.
+ *
+ * @ingroup mouse
+ */
+typedef struct GhosttyMouseEvent *GhosttyMouseEvent;
+
+/**
+ * Mouse event action type.
+ *
+ * @ingroup mouse
+ */
+typedef enum {
+  /** Mouse button was pressed. */
+  GHOSTTY_MOUSE_ACTION_PRESS = 0,
+
+  /** Mouse button was released. */
+  GHOSTTY_MOUSE_ACTION_RELEASE = 1,
+
+  /** Mouse moved. */
+  GHOSTTY_MOUSE_ACTION_MOTION = 2,
+} GhosttyMouseAction;
+
+/**
+ * Mouse button identity.
+ *
+ * @ingroup mouse
+ */
+typedef enum {
+  GHOSTTY_MOUSE_BUTTON_UNKNOWN = 0,
+  GHOSTTY_MOUSE_BUTTON_LEFT = 1,
+  GHOSTTY_MOUSE_BUTTON_RIGHT = 2,
+  GHOSTTY_MOUSE_BUTTON_MIDDLE = 3,
+  GHOSTTY_MOUSE_BUTTON_FOUR = 4,
+  GHOSTTY_MOUSE_BUTTON_FIVE = 5,
+  GHOSTTY_MOUSE_BUTTON_SIX = 6,
+  GHOSTTY_MOUSE_BUTTON_SEVEN = 7,
+  GHOSTTY_MOUSE_BUTTON_EIGHT = 8,
+  GHOSTTY_MOUSE_BUTTON_NINE = 9,
+  GHOSTTY_MOUSE_BUTTON_TEN = 10,
+  GHOSTTY_MOUSE_BUTTON_ELEVEN = 11,
+} GhosttyMouseButton;
+
+/**
+ * Mouse position in surface-space pixels.
+ *
+ * @ingroup mouse
+ */
+typedef struct {
+  float x;
+  float y;
+} GhosttyMousePosition;
+
+/**
+ * Create a new mouse event instance.
+ *
+ * @param allocator Pointer to allocator, or NULL to use the default allocator
+ * @param event Pointer to store the created event handle
+ * @return GHOSTTY_SUCCESS on success, or an error code on failure
+ *
+ * @ingroup mouse
+ */
+GhosttyResult ghostty_mouse_event_new(const GhosttyAllocator *allocator,
+                                      GhosttyMouseEvent *event);
+
+/**
+ * Free a mouse event instance.
+ *
+ * @param event The mouse event handle to free (may be NULL)
+ *
+ * @ingroup mouse
+ */
+void ghostty_mouse_event_free(GhosttyMouseEvent event);
+
+/**
+ * Set the event action.
+ *
+ * @param event The event handle, must not be NULL
+ * @param action The action to set
+ *
+ * @ingroup mouse
+ */
+void ghostty_mouse_event_set_action(GhosttyMouseEvent event,
+                                    GhosttyMouseAction action);
+
+/**
+ * Get the event action.
+ *
+ * @param event The event handle, must not be NULL
+ * @return The event action
+ *
+ * @ingroup mouse
+ */
+GhosttyMouseAction ghostty_mouse_event_get_action(GhosttyMouseEvent event);
+
+/**
+ * Set the event button.
+ *
+ * This sets a concrete button identity for the event.
+ * To represent "no button" (for motion events), use
+ * ghostty_mouse_event_clear_button().
+ *
+ * @param event The event handle, must not be NULL
+ * @param button The button to set
+ *
+ * @ingroup mouse
+ */
+void ghostty_mouse_event_set_button(GhosttyMouseEvent event,
+                                    GhosttyMouseButton button);
+
+/**
+ * Clear the event button.
+ *
+ * This sets the event button to "none".
+ *
+ * @param event The event handle, must not be NULL
+ *
+ * @ingroup mouse
+ */
+void ghostty_mouse_event_clear_button(GhosttyMouseEvent event);
+
+/**
+ * Get the event button.
+ *
+ * @param event The event handle, must not be NULL
+ * @param out_button Output pointer for the button value (may be NULL)
+ * @return true if a button is set, false if no button is set
+ *
+ * @ingroup mouse
+ */
+bool ghostty_mouse_event_get_button(GhosttyMouseEvent event,
+                                    GhosttyMouseButton *out_button);
+
+/**
+ * Set keyboard modifiers held during the event.
+ *
+ * @param event The event handle, must not be NULL
+ * @param mods Modifier bitmask
+ *
+ * @ingroup mouse
+ */
+void ghostty_mouse_event_set_mods(GhosttyMouseEvent event,
+                                  GhosttyMods mods);
+
+/**
+ * Get keyboard modifiers held during the event.
+ *
+ * @param event The event handle, must not be NULL
+ * @return Modifier bitmask
+ *
+ * @ingroup mouse
+ */
+GhosttyMods ghostty_mouse_event_get_mods(GhosttyMouseEvent event);
+
+/**
+ * Set the event position in surface-space pixels.
+ *
+ * @param event The event handle, must not be NULL
+ * @param position The position to set
+ *
+ * @ingroup mouse
+ */
+void ghostty_mouse_event_set_position(GhosttyMouseEvent event,
+                                      GhosttyMousePosition position);
+
+/**
+ * Get the event position in surface-space pixels.
+ *
+ * @param event The event handle, must not be NULL
+ * @return The current event position
+ *
+ * @ingroup mouse
+ */
+GhosttyMousePosition ghostty_mouse_event_get_position(GhosttyMouseEvent event);
+
+#endif /* GHOSTTY_VT_MOUSE_EVENT_H */

src/input/mouse.zig

@@ -4,7 +4,7 @@ const std = @import("std");
 /// from ButtonState because button state is simply the current state
 /// of a mouse button but an action is something that triggers via
 /// an GUI event and supports more.
-pub const Action = enum { press, release, motion };
+pub const Action = enum(c_int) { press, release, motion };
 
 /// The state of a mouse button.
 ///

src/lib_vt.zig

@@ -126,6 +126,23 @@ comptime {
         @export(&c.key_encoder_setopt, .{ .name = "ghostty_key_encoder_setopt" });
         @export(&c.key_encoder_setopt_from_terminal, .{ .name = "ghostty_key_encoder_setopt_from_terminal" });
         @export(&c.key_encoder_encode, .{ .name = "ghostty_key_encoder_encode" });
+        @export(&c.mouse_event_new, .{ .name = "ghostty_mouse_event_new" });
+        @export(&c.mouse_event_free, .{ .name = "ghostty_mouse_event_free" });
+        @export(&c.mouse_event_set_action, .{ .name = "ghostty_mouse_event_set_action" });
+        @export(&c.mouse_event_get_action, .{ .name = "ghostty_mouse_event_get_action" });
+        @export(&c.mouse_event_set_button, .{ .name = "ghostty_mouse_event_set_button" });
+        @export(&c.mouse_event_clear_button, .{ .name = "ghostty_mouse_event_clear_button" });
+        @export(&c.mouse_event_get_button, .{ .name = "ghostty_mouse_event_get_button" });
+        @export(&c.mouse_event_set_mods, .{ .name = "ghostty_mouse_event_set_mods" });
+        @export(&c.mouse_event_get_mods, .{ .name = "ghostty_mouse_event_get_mods" });
+        @export(&c.mouse_event_set_position, .{ .name = "ghostty_mouse_event_set_position" });
+        @export(&c.mouse_event_get_position, .{ .name = "ghostty_mouse_event_get_position" });
+        @export(&c.mouse_encoder_new, .{ .name = "ghostty_mouse_encoder_new" });
+        @export(&c.mouse_encoder_free, .{ .name = "ghostty_mouse_encoder_free" });
+        @export(&c.mouse_encoder_setopt, .{ .name = "ghostty_mouse_encoder_setopt" });
+        @export(&c.mouse_encoder_setopt_from_terminal, .{ .name = "ghostty_mouse_encoder_setopt_from_terminal" });
+        @export(&c.mouse_encoder_reset, .{ .name = "ghostty_mouse_encoder_reset" });
+        @export(&c.mouse_encoder_encode, .{ .name = "ghostty_mouse_encoder_encode" });
         @export(&c.osc_new, .{ .name = "ghostty_osc_new" });
         @export(&c.osc_free, .{ .name = "ghostty_osc_free" });
         @export(&c.osc_next, .{ .name = "ghostty_osc_next" });

src/terminal/c/main.zig

@@ -3,6 +3,8 @@ pub const formatter = @import("formatter.zig");
 pub const osc = @import("osc.zig");
 pub const key_event = @import("key_event.zig");
 pub const key_encode = @import("key_encode.zig");
+pub const mouse_event = @import("mouse_event.zig");
+pub const mouse_encode = @import("mouse_encode.zig");
 pub const paste = @import("paste.zig");
 pub const sgr = @import("sgr.zig");
 pub const terminal = @import("terminal.zig");
@@ -58,6 +60,25 @@ pub const key_encoder_setopt = key_encode.setopt;
 pub const key_encoder_setopt_from_terminal = key_encode.setopt_from_terminal;
 pub const key_encoder_encode = key_encode.encode;
 
+pub const mouse_event_new = mouse_event.new;
+pub const mouse_event_free = mouse_event.free;
+pub const mouse_event_set_action = mouse_event.set_action;
+pub const mouse_event_get_action = mouse_event.get_action;
+pub const mouse_event_set_button = mouse_event.set_button;
+pub const mouse_event_clear_button = mouse_event.clear_button;
+pub const mouse_event_get_button = mouse_event.get_button;
+pub const mouse_event_set_mods = mouse_event.set_mods;
+pub const mouse_event_get_mods = mouse_event.get_mods;
+pub const mouse_event_set_position = mouse_event.set_position;
+pub const mouse_event_get_position = mouse_event.get_position;
+
+pub const mouse_encoder_new = mouse_encode.new;
+pub const mouse_encoder_free = mouse_encode.free;
+pub const mouse_encoder_setopt = mouse_encode.setopt;
+pub const mouse_encoder_setopt_from_terminal = mouse_encode.setopt_from_terminal;
+pub const mouse_encoder_reset = mouse_encode.reset;
+pub const mouse_encoder_encode = mouse_encode.encode;
+
 pub const paste_is_safe = paste.is_safe;
 
 pub const terminal_new = terminal.new;
@@ -73,6 +94,8 @@ test {
     _ = osc;
     _ = key_event;
     _ = key_encode;
+    _ = mouse_event;
+    _ = mouse_encode;
     _ = paste;
     _ = sgr;
     _ = terminal;

src/terminal/c/mouse_encode.zig

@@ -0,0 +1,529 @@
+const std = @import("std");
+const Allocator = std.mem.Allocator;
+const testing = std.testing;
+const lib_alloc = @import("../../lib/allocator.zig");
+const CAllocator = lib_alloc.Allocator;
+const input_mouse_encode = @import("../../input/mouse_encode.zig");
+const renderer_size = @import("../../renderer/size.zig");
+const point = @import("../point.zig");
+const terminal_mouse = @import("../mouse.zig");
+const mouse_event = @import("mouse_event.zig");
+const Result = @import("result.zig").Result;
+const Event = mouse_event.Event;
+const Terminal = @import("terminal.zig").Terminal;
+
+const log = std.log.scoped(.mouse_encode);
+
+/// Wrapper around mouse encoding options that tracks the allocator for C API usage.
+const MouseEncoderWrapper = struct {
+    opts: input_mouse_encode.Options,
+    track_last_cell: bool = false,
+    last_cell: ?point.Coordinate = null,
+    alloc: Allocator,
+};
+
+/// C: GhosttyMouseEncoder
+pub const Encoder = ?*MouseEncoderWrapper;
+
+/// C: GhosttyMouseTrackingMode
+pub const TrackingMode = terminal_mouse.Event;
+
+/// C: GhosttyMouseFormat
+pub const Format = terminal_mouse.Format;
+
+/// C: GhosttyMouseEncoderSize
+pub const Size = extern struct {
+    size: usize = @sizeOf(Size),
+    screen_width: u32,
+    screen_height: u32,
+    cell_width: u32,
+    cell_height: u32,
+    padding_top: u32,
+    padding_bottom: u32,
+    padding_right: u32,
+    padding_left: u32,
+
+    fn toRenderer(self: Size) ?renderer_size.Size {
+        if (self.cell_width == 0 or self.cell_height == 0) return null;
+        return .{
+            .screen = .{
+                .width = self.screen_width,
+                .height = self.screen_height,
+            },
+            .cell = .{
+                .width = self.cell_width,
+                .height = self.cell_height,
+            },
+            .padding = .{
+                .top = self.padding_top,
+                .bottom = self.padding_bottom,
+                .right = self.padding_right,
+                .left = self.padding_left,
+            },
+        };
+    }
+};
+
+/// C: GhosttyMouseEncoderOption
+pub const Option = enum(c_int) {
+    event = 0,
+    format = 1,
+    size = 2,
+    any_button_pressed = 3,
+    track_last_cell = 4,
+
+    /// Input type expected for setting the option.
+    pub fn InType(comptime self: Option) type {
+        return switch (self) {
+            .event => TrackingMode,
+            .format => Format,
+            .size => Size,
+            .any_button_pressed,
+            .track_last_cell,
+            => bool,
+        };
+    }
+};
+
+pub fn new(
+    alloc_: ?*const CAllocator,
+    result: *Encoder,
+) callconv(.c) Result {
+    const alloc = lib_alloc.default(alloc_);
+    const ptr = alloc.create(MouseEncoderWrapper) catch
+        return .out_of_memory;
+    ptr.* = .{
+        .opts = .{ .size = defaultSize() },
+        .alloc = alloc,
+    };
+    result.* = ptr;
+    return .success;
+}
+
+pub fn free(encoder_: Encoder) callconv(.c) void {
+    const wrapper = encoder_ orelse return;
+    const alloc = wrapper.alloc;
+    alloc.destroy(wrapper);
+}
+
+pub fn setopt(
+    encoder_: Encoder,
+    option: Option,
+    value: ?*const anyopaque,
+) callconv(.c) void {
+    if (comptime std.debug.runtime_safety) {
+        _ = std.meta.intToEnum(Option, @intFromEnum(option)) catch {
+            log.warn("setopt invalid option value={d}", .{@intFromEnum(option)});
+            return;
+        };
+    }
+
+    return switch (option) {
+        inline else => |comptime_option| setoptTyped(
+            encoder_,
+            comptime_option,
+            @ptrCast(@alignCast(value orelse return)),
+        ),
+    };
+}
+
+fn setoptTyped(
+    encoder_: Encoder,
+    comptime option: Option,
+    value: *const option.InType(),
+) void {
+    const wrapper = encoder_.?;
+    switch (option) {
+        .event => {
+            if (comptime std.debug.runtime_safety) {
+                _ = std.meta.intToEnum(TrackingMode, @intFromEnum(value.*)) catch {
+                    log.warn("setopt invalid TrackingMode value={d}", .{@intFromEnum(value.*)});
+                    return;
+                };
+            }
+
+            if (wrapper.opts.event != value.*) wrapper.last_cell = null;
+            wrapper.opts.event = value.*;
+        },
+
+        .format => {
+            if (comptime std.debug.runtime_safety) {
+                _ = std.meta.intToEnum(Format, @intFromEnum(value.*)) catch {
+                    log.warn("setopt invalid Format value={d}", .{@intFromEnum(value.*)});
+                    return;
+                };
+            }
+
+            if (wrapper.opts.format != value.*) wrapper.last_cell = null;
+            wrapper.opts.format = value.*;
+        },
+
+        .size => {
+            if (value.size < @sizeOf(Size)) {
+                log.warn("setopt size struct too small size={d} expected>={d}", .{
+                    value.size,
+                    @sizeOf(Size),
+                });
+                return;
+            }
+
+            wrapper.opts.size = value.toRenderer() orelse {
+                log.warn("setopt invalid size values (cell width and height must be non-zero)", .{});
+                return;
+            };
+            wrapper.last_cell = null;
+        },
+
+        .any_button_pressed => wrapper.opts.any_button_pressed = value.*,
+
+        .track_last_cell => {
+            wrapper.track_last_cell = value.*;
+            if (!value.*) wrapper.last_cell = null;
+        },
+    }
+}
+
+pub fn setopt_from_terminal(
+    encoder_: Encoder,
+    terminal_: Terminal,
+) callconv(.c) void {
+    const wrapper = encoder_ orelse return;
+    const t = terminal_ orelse return;
+    wrapper.opts.event = t.flags.mouse_event;
+    wrapper.opts.format = t.flags.mouse_format;
+    wrapper.last_cell = null;
+}
+
+pub fn reset(encoder_: Encoder) callconv(.c) void {
+    const wrapper = encoder_ orelse return;
+    wrapper.last_cell = null;
+}
+
+pub fn encode(
+    encoder_: Encoder,
+    event_: Event,
+    out_: ?[*]u8,
+    out_len: usize,
+    out_written: *usize,
+) callconv(.c) Result {
+    const wrapper = encoder_ orelse return .invalid_value;
+    const event = event_ orelse return .invalid_value;
+
+    const prev_last_cell = wrapper.last_cell;
+
+    var opts = wrapper.opts;
+    opts.last_cell = if (wrapper.track_last_cell) &wrapper.last_cell else null;
+
+    var writer: std.Io.Writer = .fixed(if (out_) |out| out[0..out_len] else &.{});
+    input_mouse_encode.encode(
+        &writer,
+        event.event,
+        opts,
+    ) catch |err| switch (err) {
+        error.WriteFailed => {
+            // Failed writes should not mutate motion dedupe state because no
+            // complete sequence was produced.
+            wrapper.last_cell = prev_last_cell;
+
+            // Use a discarding writer to count how much space we would have needed.
+            var count_last_cell = prev_last_cell;
+            var count_opts = wrapper.opts;
+            count_opts.last_cell = if (wrapper.track_last_cell) &count_last_cell else null;
+
+            var discarding: std.Io.Writer.Discarding = .init(&.{});
+            input_mouse_encode.encode(
+                &discarding.writer,
+                event.event,
+                count_opts,
+            ) catch unreachable;
+
+            // Discarding always uses a u64. If we're on 32-bit systems
+            // we cast down. We should make this safer in the future.
+            out_written.* = @intCast(discarding.count);
+            return .out_of_memory;
+        },
+    };
+
+    out_written.* = writer.end;
+    return .success;
+}
+
+fn defaultSize() renderer_size.Size {
+    return .{
+        .screen = .{ .width = 1, .height = 1 },
+        .cell = .{ .width = 1, .height = 1 },
+        .padding = .{},
+    };
+}
+
+fn testSize() Size {
+    return .{
+        .size = @sizeOf(Size),
+        .screen_width = 1_000,
+        .screen_height = 1_000,
+        .cell_width = 1,
+        .cell_height = 1,
+        .padding_top = 0,
+        .padding_bottom = 0,
+        .padding_right = 0,
+        .padding_left = 0,
+    };
+}
+
+test "alloc" {
+    var e: Encoder = undefined;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &e,
+    ));
+    free(e);
+}
+
+test "setopt" {
+    var e: Encoder = undefined;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &e,
+    ));
+    defer free(e);
+
+    const event_mode: TrackingMode = .any;
+    setopt(e, .event, &event_mode);
+    try testing.expectEqual(TrackingMode.any, e.?.opts.event);
+
+    const format_mode: Format = .sgr;
+    setopt(e, .format, &format_mode);
+    try testing.expectEqual(Format.sgr, e.?.opts.format);
+
+    const size = testSize();
+    setopt(e, .size, &size);
+    try testing.expectEqual(size.screen_width, e.?.opts.size.screen.width);
+    try testing.expectEqual(size.screen_height, e.?.opts.size.screen.height);
+
+    const any_button_pressed = true;
+    setopt(e, .any_button_pressed, &any_button_pressed);
+    try testing.expect(e.?.opts.any_button_pressed);
+
+    const track_last_cell = true;
+    setopt(e, .track_last_cell, &track_last_cell);
+    try testing.expect(e.?.track_last_cell);
+}
+
+test "setopt_from_terminal" {
+    const terminal_c = @import("terminal.zig");
+
+    var e: Encoder = undefined;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &e,
+    ));
+    defer free(e);
+
+    var t: Terminal = undefined;
+    try testing.expectEqual(Result.success, terminal_c.new(
+        &lib_alloc.test_allocator,
+        &t,
+        .{ .cols = 80, .rows = 24, .max_scrollback = 0 },
+    ));
+    defer terminal_c.free(t);
+
+    const event_mode: TrackingMode = .any;
+    setopt(e, .event, &event_mode);
+    const format_mode: Format = .sgr;
+    setopt(e, .format, &format_mode);
+
+    setopt_from_terminal(e, t);
+    try testing.expectEqual(TrackingMode.none, e.?.opts.event);
+    try testing.expectEqual(Format.x10, e.?.opts.format);
+}
+
+test "setopt_from_terminal null" {
+    setopt_from_terminal(null, null);
+
+    const terminal_c = @import("terminal.zig");
+    var t: Terminal = undefined;
+    try testing.expectEqual(Result.success, terminal_c.new(
+        &lib_alloc.test_allocator,
+        &t,
+        .{ .cols = 80, .rows = 24, .max_scrollback = 0 },
+    ));
+    defer terminal_c.free(t);
+    setopt_from_terminal(null, t);
+
+    var e: Encoder = undefined;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &e,
+    ));
+    defer free(e);
+    setopt_from_terminal(e, null);
+}
+
+test "encode: sgr press left" {
+    var encoder: Encoder = undefined;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &encoder,
+    ));
+    defer free(encoder);
+
+    const event_mode: TrackingMode = .any;
+    setopt(encoder, .event, &event_mode);
+    const format_mode: Format = .sgr;
+    setopt(encoder, .format, &format_mode);
+    const size = testSize();
+    setopt(encoder, .size, &size);
+
+    var event: Event = undefined;
+    try testing.expectEqual(Result.success, mouse_event.new(
+        &lib_alloc.test_allocator,
+        &event,
+    ));
+    defer mouse_event.free(event);
+
+    mouse_event.set_action(event, .press);
+    mouse_event.set_button(event, .left);
+    mouse_event.set_position(event, .{ .x = 0, .y = 0 });
+
+    var required: usize = 0;
+    try testing.expectEqual(Result.out_of_memory, encode(
+        encoder,
+        event,
+        null,
+        0,
+        &required,
+    ));
+    try testing.expectEqual(@as(usize, 9), required);
+
+    var buf: [32]u8 = undefined;
+    var written: usize = 0;
+    try testing.expectEqual(Result.success, encode(
+        encoder,
+        event,
+        &buf,
+        buf.len,
+        &written,
+    ));
+    try testing.expectEqual(required, written);
+    try testing.expectEqualStrings("\x1b[<0;1;1M", buf[0..written]);
+}
+
+test "encode: motion dedupe and reset" {
+    var encoder: Encoder = undefined;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &encoder,
+    ));
+    defer free(encoder);
+
+    const event_mode: TrackingMode = .any;
+    setopt(encoder, .event, &event_mode);
+    const format_mode: Format = .sgr;
+    setopt(encoder, .format, &format_mode);
+    const size = testSize();
+    setopt(encoder, .size, &size);
+    const track_last_cell = true;
+    setopt(encoder, .track_last_cell, &track_last_cell);
+
+    var event: Event = undefined;
+    try testing.expectEqual(Result.success, mouse_event.new(
+        &lib_alloc.test_allocator,
+        &event,
+    ));
+    defer mouse_event.free(event);
+
+    mouse_event.set_action(event, .motion);
+    mouse_event.set_button(event, .left);
+    mouse_event.set_position(event, .{ .x = 5, .y = 6 });
+
+    {
+        var buf: [32]u8 = undefined;
+        var written: usize = 0;
+        try testing.expectEqual(Result.success, encode(
+            encoder,
+            event,
+            &buf,
+            buf.len,
+            &written,
+        ));
+        try testing.expect(written > 0);
+    }
+
+    {
+        var buf: [32]u8 = undefined;
+        var written: usize = 0;
+        try testing.expectEqual(Result.success, encode(
+            encoder,
+            event,
+            &buf,
+            buf.len,
+            &written,
+        ));
+        try testing.expectEqual(@as(usize, 0), written);
+    }
+
+    reset(encoder);
+
+    {
+        var buf: [32]u8 = undefined;
+        var written: usize = 0;
+        try testing.expectEqual(Result.success, encode(
+            encoder,
+            event,
+            &buf,
+            buf.len,
+            &written,
+        ));
+        try testing.expect(written > 0);
+    }
+}
+
+test "encode: querying required size doesn't update dedupe state" {
+    var encoder: Encoder = undefined;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &encoder,
+    ));
+    defer free(encoder);
+
+    const event_mode: TrackingMode = .any;
+    setopt(encoder, .event, &event_mode);
+    const format_mode: Format = .sgr;
+    setopt(encoder, .format, &format_mode);
+    const size = testSize();
+    setopt(encoder, .size, &size);
+    const track_last_cell = true;
+    setopt(encoder, .track_last_cell, &track_last_cell);
+
+    var event: Event = undefined;
+    try testing.expectEqual(Result.success, mouse_event.new(
+        &lib_alloc.test_allocator,
+        &event,
+    ));
+    defer mouse_event.free(event);
+
+    mouse_event.set_action(event, .motion);
+    mouse_event.set_button(event, .left);
+    mouse_event.set_position(event, .{ .x = 5, .y = 6 });
+
+    var required: usize = 0;
+    try testing.expectEqual(Result.out_of_memory, encode(
+        encoder,
+        event,
+        null,
+        0,
+        &required,
+    ));
+    try testing.expect(required > 0);
+
+    var buf: [32]u8 = undefined;
+    var written: usize = 0;
+    try testing.expectEqual(Result.success, encode(
+        encoder,
+        event,
+        &buf,
+        buf.len,
+        &written,
+    ));
+    try testing.expect(written > 0);
+}

src/terminal/c/mouse_event.zig

@@ -0,0 +1,155 @@
+const std = @import("std");
+const Allocator = std.mem.Allocator;
+const testing = std.testing;
+const lib_alloc = @import("../../lib/allocator.zig");
+const CAllocator = lib_alloc.Allocator;
+const key = @import("../../input/key.zig");
+const mouse = @import("../../input/mouse.zig");
+const mouse_encode = @import("../../input/mouse_encode.zig");
+const Result = @import("result.zig").Result;
+
+const log = std.log.scoped(.mouse_event);
+
+/// Wrapper around mouse event that tracks the allocator for C API usage.
+const MouseEventWrapper = struct {
+    event: mouse_encode.Event = .{},
+    alloc: Allocator,
+};
+
+/// C: GhosttyMouseEvent
+pub const Event = ?*MouseEventWrapper;
+
+/// C: GhosttyMouseAction
+pub const Action = mouse.Action;
+
+/// C: GhosttyMouseButton
+pub const Button = mouse.Button;
+
+/// C: GhosttyMousePosition
+pub const Position = mouse_encode.Event.Pos;
+
+/// C: GhosttyMods
+pub const Mods = key.Mods;
+
+pub fn new(
+    alloc_: ?*const CAllocator,
+    result: *Event,
+) callconv(.c) Result {
+    const alloc = lib_alloc.default(alloc_);
+    const ptr = alloc.create(MouseEventWrapper) catch
+        return .out_of_memory;
+    ptr.* = .{ .alloc = alloc };
+    result.* = ptr;
+    return .success;
+}
+
+pub fn free(event_: Event) callconv(.c) void {
+    const wrapper = event_ orelse return;
+    const alloc = wrapper.alloc;
+    alloc.destroy(wrapper);
+}
+
+pub fn set_action(event_: Event, action: Action) callconv(.c) void {
+    if (comptime std.debug.runtime_safety) {
+        _ = std.meta.intToEnum(Action, @intFromEnum(action)) catch {
+            log.warn("set_action invalid action value={d}", .{@intFromEnum(action)});
+            return;
+        };
+    }
+
+    event_.?.event.action = action;
+}
+
+pub fn get_action(event_: Event) callconv(.c) Action {
+    return event_.?.event.action;
+}
+
+pub fn set_button(event_: Event, button: Button) callconv(.c) void {
+    if (comptime std.debug.runtime_safety) {
+        _ = std.meta.intToEnum(Button, @intFromEnum(button)) catch {
+            log.warn("set_button invalid button value={d}", .{@intFromEnum(button)});
+            return;
+        };
+    }
+
+    event_.?.event.button = button;
+}
+
+pub fn clear_button(event_: Event) callconv(.c) void {
+    event_.?.event.button = null;
+}
+
+pub fn get_button(event_: Event, out: ?*Button) callconv(.c) bool {
+    if (event_.?.event.button) |button| {
+        if (out) |ptr| ptr.* = button;
+        return true;
+    }
+
+    return false;
+}
+
+pub fn set_mods(event_: Event, mods: Mods) callconv(.c) void {
+    event_.?.event.mods = mods;
+}
+
+pub fn get_mods(event_: Event) callconv(.c) Mods {
+    return event_.?.event.mods;
+}
+
+pub fn set_position(event_: Event, pos: Position) callconv(.c) void {
+    event_.?.event.pos = pos;
+}
+
+pub fn get_position(event_: Event) callconv(.c) Position {
+    return event_.?.event.pos;
+}
+
+test "alloc" {
+    var e: Event = undefined;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &e,
+    ));
+    free(e);
+}
+
+test "free null" {
+    free(null);
+}
+
+test "set/get" {
+    var e: Event = undefined;
+    try testing.expectEqual(Result.success, new(
+        &lib_alloc.test_allocator,
+        &e,
+    ));
+    defer free(e);
+
+    // Action
+    set_action(e, .motion);
+    try testing.expectEqual(Action.motion, get_action(e));
+
+    // Button
+    set_button(e, .left);
+    var button: Button = .unknown;
+    try testing.expect(get_button(e, &button));
+    try testing.expectEqual(Button.left, button);
+    try testing.expect(get_button(e, null));
+
+    clear_button(e);
+    try testing.expect(!get_button(e, &button));
+
+    // Mods
+    const mods: Mods = .{ .shift = true, .ctrl = true };
+    set_mods(e, mods);
+    const got_mods = get_mods(e);
+    try testing.expect(got_mods.shift);
+    try testing.expect(got_mods.ctrl);
+    try testing.expect(!got_mods.alt);
+
+    // Position
+    set_position(e, .{ .x = 12.5, .y = -4.0 });
+    const pos = get_position(e);
+    try testing.expectEqual(@as(f32, 12.5), pos.x);
+    try testing.expectEqual(@as(f32, -4.0), pos.y);
+}