renderer: emit scrollbar apprt event when scrollbar changes

src/Surface.zig

@@ -983,6 +983,8 @@ pub fn handleMessage(self: *Surface, msg: Message) !void {
 
         .renderer_health => |health| self.updateRendererHealth(health),
 
+        .scrollbar => |scrollbar| self.updateScrollbar(scrollbar),
+
         .report_color_scheme => |force| self.reportColorScheme(force),
 
         .present_surface => try self.presentSurface(),
@@ -1459,6 +1461,17 @@ fn updateRendererHealth(self: *Surface, health: rendererpkg.Health) void {
     };
 }
 
+/// Called when the scrollbar state changes.
+fn updateScrollbar(self: *Surface, scrollbar: terminal.Scrollbar) void {
+    _ = self.rt_app.performAction(
+        .{ .surface = self },
+        .scrollbar,
+        scrollbar,
+    ) catch |err| {
+        log.warn("failed to notify app of scrollbar change err={}", .{err});
+    };
+}
+
 /// This should be called anytime `config_conditional_state` changes
 /// so that the apprt can reload the configuration.
 fn notifyConfigConditionalState(self: *Surface) void {

src/apprt/action.zig

@@ -164,6 +164,9 @@ pub const Action = union(Key) {
     /// The cell size has changed to the given dimensions in pixels.
     cell_size: CellSize,
 
+    /// The scrollbar is updating.
+    scrollbar: terminal.Scrollbar,
+
     /// The target should be re-rendered. This usually has a specific
     /// surface target but if the app is targeted then all active
     /// surfaces should be redrawn.
@@ -324,6 +327,7 @@ pub const Action = union(Key) {
         reset_window_size,
         initial_size,
         cell_size,
+        scrollbar,
         render,
         inspector,
         show_gtk_inspector,

src/apprt/surface.zig

@@ -104,6 +104,9 @@ pub const Message = union(enum) {
     /// of the command.
     stop_command: ?u8,
 
+    /// The scrollbar state changed for the surface.
+    scrollbar: terminal.Scrollbar,
+
     pub const ReportTitleStyle = enum {
         csi_21_t,
 

src/renderer/generic.zig

@@ -1314,6 +1314,15 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
             self.draw_mutex.lock();
             defer self.draw_mutex.unlock();
 
+            // After the graphics API is complete (so we defer) we want to
+            // update our scrollbar state.
+            defer if (self.scrollbar_dirty) {
+                self.scrollbar_dirty = false;
+                _ = self.surface_mailbox.push(.{
+                    .scrollbar = self.scrollbar,
+                }, .{ .forever = {} });
+            };
+
             // Let our graphics API do any bookkeeping, etc.
             // that it needs to do before / after `drawFrame`.
             self.api.drawFrameStart();

src/terminal/PageList.zig

@@ -2110,6 +2110,21 @@ pub const Scrollbar = struct {
         .len = 0,
     };
 
+    // Sync with: ghostty_action_scrollbar_s
+    pub const C = extern struct {
+        total: u64,
+        offset: u64,
+        len: u64,
+    };
+
+    pub fn cval(self: Scrollbar) C {
+        return .{
+            .total = @intCast(self.total),
+            .offset = @intCast(self.offset),
+            .len = @intCast(self.len),
+        };
+    }
+
     /// Comparison for scrollbars.
     pub fn eql(self: Scrollbar, other: Scrollbar) bool {
         return self.total == other.total and