embedded: add ghostty_surface_is_alternate_screen

include/ghostty.h

@@ -1637,6 +1637,9 @@ GHOSTTY_API void ghostty_surface_set_pty_tee_cb(ghostty_surface_t,
                                                 void* userdata);
 
 GHOSTTY_API bool ghostty_surface_mouse_captured(ghostty_surface_t);
+// cmux fork: true when the terminal's active screen is the alternate screen.
+// Takes the renderer state mutex, same as ghostty_surface_mouse_captured.
+GHOSTTY_API bool ghostty_surface_is_alternate_screen(ghostty_surface_t);
 GHOSTTY_API bool ghostty_surface_mouse_button(ghostty_surface_t,
                                                  ghostty_input_mouse_state_e,
                                                  ghostty_input_mouse_button_e,

src/Surface.zig

@@ -5650,6 +5650,14 @@ pub fn mouseCaptured(self: *Surface) bool {
     return self.io.terminal.flags.mouse_event != .none;
 }
 
+/// Returns true if the terminal's active screen is the alternate screen
+/// (e.g. a full-screen TUI such as vim, less, or htop is running).
+pub fn isAlternateScreen(self: *Surface) bool {
+    self.renderer_state.mutex.lockUncancelable(global.io());
+    defer self.renderer_state.mutex.unlock(global.io());
+    return self.io.terminal.screens.active_key == .alternate;
+}
+
 /// Called for mouse button press/release events. This will return true
 /// if the mouse event was consumed in some way (i.e. the program is capturing
 /// mouse events). If the event was not consumed, then false is returned.

src/apprt/embedded.zig

@@ -5014,6 +5014,12 @@ pub const CAPI = struct {
         return surface.core_surface.mouseCaptured();
     }
 
+    /// Returns true if the terminal's active screen is the alternate
+    /// screen (cmux-specific).
+    export fn ghostty_surface_is_alternate_screen(surface: *Surface) bool {
+        return surface.core_surface.isAlternateScreen();
+    }
+
     /// Tell the surface that it needs to schedule a render
     export fn ghostty_surface_mouse_button(
         surface: *Surface,