feat: implement scroll-to-bottom on output option

Implements the `output` option for the `scroll-to-bottom` configuration,
which scrolls the viewport to the bottom when new lines are printed.

Co-Authored-By: Sachin <sachinbeniwal0101@gmail.com>

src/config/Config.zig

@@ -835,7 +835,7 @@ palette: Palette = .{},
 ///   anything but modifiers or keybinds that are processed by Ghostty).
 ///
 /// - `output` If set, scroll the surface to the bottom if there is new data
-///   to display. (Currently unimplemented.)
+///   to display (e.g., when new lines are printed to the terminal).
 ///
 /// The default is `keystroke, no-output`.
 @"scroll-to-bottom": ScrollToBottom = .default,

src/termio/Termio.zig

@@ -165,6 +165,7 @@ pub const DerivedConfig = struct {
     osc_color_report_format: configpkg.Config.OSCColorReportFormat,
     clipboard_write: configpkg.ClipboardAccess,
     enquiry_response: []const u8,
+    scroll_to_bottom: configpkg.Config.ScrollToBottom,
 
     pub fn init(
         alloc_gpa: Allocator,
@@ -185,6 +186,7 @@ pub const DerivedConfig = struct {
             .osc_color_report_format = config.@"osc-color-report-format",
             .clipboard_write = config.@"clipboard-write",
             .enquiry_response = try alloc.dupe(u8, config.@"enquiry-response"),
+            .scroll_to_bottom = config.@"scroll-to-bottom",
 
             // This has to be last so that we copy AFTER the arena allocations
             // above happen (Zig assigns in order).
@@ -279,6 +281,7 @@ pub fn init(self: *Termio, alloc: Allocator, opts: termio.Options) !void {
         .enquiry_response = opts.config.enquiry_response,
         .default_cursor_style = opts.config.cursor_style,
         .default_cursor_blink = opts.config.cursor_blink,
+        .scroll_to_bottom_on_output = opts.config.scroll_to_bottom.output,
     };
 
     const thread_enter_state = try ThreadEnterState.create(

src/termio/stream_handler.zig

@@ -56,6 +56,9 @@ pub const StreamHandler = struct {
     /// The clipboard write access configuration.
     clipboard_write: configpkg.ClipboardAccess,
 
+    /// The scroll-to-bottom behavior configuration.
+    scroll_to_bottom_on_output: bool,
+
     //---------------------------------------------------------------
     // Internal state
 
@@ -112,6 +115,7 @@ pub const StreamHandler = struct {
         self.enquiry_response = config.enquiry_response;
         self.default_cursor_style = config.cursor_style;
         self.default_cursor_blink = config.cursor_blink;
+        self.scroll_to_bottom_on_output = config.scroll_to_bottom.output;
 
         // If our cursor is the default, then we update it immediately.
         if (self.default_cursor) self.setCursorStyle(.default) catch |err| {
@@ -575,6 +579,11 @@ pub const StreamHandler = struct {
         // Small optimization: call index instead of linefeed because they're
         // identical and this avoids one layer of function call overhead.
         try self.terminal.index();
+
+        // If configured, scroll to bottom on output so the user sees new content
+        if (self.scroll_to_bottom_on_output) {
+            try self.terminal.scrollViewport(.bottom);
+        }
     }
 
     pub inline fn reverseIndex(self: *StreamHandler) !void {