feat: implement scroll-to-bottom on output option (#9938)

Closes #8408

src/Surface.zig

@@ -1174,7 +1174,7 @@ fn selectionScrollTick(self: *Surface) !void {
     }
 
     // Scroll the viewport as required
-    try t.scrollViewport(.{ .delta = delta });
+    t.scrollViewport(.{ .delta = delta });
 
     // Next, trigger our drag behavior
     const pin = t.screens.active.pages.pin(.{
@@ -2779,7 +2779,7 @@ pub fn keyCallback(
             try self.setSelection(null);
         }
 
-        if (self.config.scroll_to_bottom.keystroke) try self.io.terminal.scrollViewport(.bottom);
+        if (self.config.scroll_to_bottom.keystroke) self.io.terminal.scrollViewport(.bottom);
 
         try self.queueRender();
     }
@@ -3532,7 +3532,7 @@ pub fn scrollCallback(
             // Modify our viewport, this requires a lock since it affects
             // rendering. We have to switch signs here because our delta
             // is negative down but our viewport is positive down.
-            try self.io.terminal.scrollViewport(.{ .delta = y.delta * -1 });
+            self.io.terminal.scrollViewport(.{ .delta = y.delta * -1 });
         }
     }
 
@@ -5063,7 +5063,7 @@ pub fn posToViewport(self: Surface, xpos: f64, ypos: f64) terminal.point.Coordin
 ///
 /// Precondition: the render_state mutex must be held.
 fn scrollToBottom(self: *Surface) !void {
-    try self.io.terminal.scrollViewport(.{ .bottom = {} });
+    self.io.terminal.scrollViewport(.{ .bottom = {} });
     try self.queueRender();
 }
 

src/config/Config.zig

@@ -906,7 +906,7 @@ 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/renderer/cursor.zig

@@ -143,7 +143,7 @@ test "cursor: always block with preedit" {
 
     // If we're scrolled though, then we don't show the cursor.
     for (0..100) |_| try term.index();
-    try term.scrollViewport(.{ .top = {} });
+    term.scrollViewport(.{ .top = {} });
     try state.update(alloc, &term);
 
     // In any bool state

src/renderer/generic.zig

@@ -125,6 +125,12 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
         scrollbar: terminal.Scrollbar,
         scrollbar_dirty: bool,
 
+        /// Tracks the last bottom-right pin of the screen to detect new output.
+        /// When the final line changes (node or y differs), new content was added.
+        /// Used for scroll-to-bottom on output feature.
+        last_bottom_node: ?usize,
+        last_bottom_y: terminal.size.CellCountInt,
+
         /// The most recent viewport matches so that we can render search
         /// matches in the visible frame. This is provided asynchronously
         /// from the search thread so we have the dirty flag to also note
@@ -563,6 +569,7 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
             colorspace: configpkg.Config.WindowColorspace,
             blending: configpkg.Config.AlphaBlending,
             background_blur: configpkg.Config.BackgroundBlur,
+            scroll_to_bottom_on_output: bool,
 
             pub fn init(
                 alloc_gpa: Allocator,
@@ -636,6 +643,7 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
                     .colorspace = config.@"window-colorspace",
                     .blending = config.@"alpha-blending",
                     .background_blur = config.@"background-blur",
+                    .scroll_to_bottom_on_output = config.@"scroll-to-bottom".output,
                     .arena = arena,
                 };
             }
@@ -699,6 +707,8 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
                 .focused = true,
                 .scrollbar = .zero,
                 .scrollbar_dirty = false,
+                .last_bottom_node = null,
+                .last_bottom_y = 0,
                 .search_matches = null,
                 .search_selected_match = null,
                 .search_matches_dirty = false,
@@ -1166,6 +1176,26 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
                     return;
                 }
 
+                // If scroll-to-bottom on output is enabled, check if the final line
+                // changed by comparing the bottom-right pin. If the node pointer or
+                // y offset changed, new content was added to the screen.
+                // Update this BEFORE we update our render state so we can
+                // draw the new scrolled data immediately.
+                if (self.config.scroll_to_bottom_on_output) scroll: {
+                    const br = state.terminal.screens.active.pages.getBottomRight(.screen) orelse break :scroll;
+
+                    // If the pin hasn't changed, then don't scroll.
+                    if (self.last_bottom_node == @intFromPtr(br.node) and
+                        self.last_bottom_y == br.y) break :scroll;
+
+                    // Update tracked pin state for next frame
+                    self.last_bottom_node = @intFromPtr(br.node);
+                    self.last_bottom_y = br.y;
+
+                    // Scroll
+                    state.terminal.scrollViewport(.bottom);
+                }
+
                 // Update our terminal state
                 try self.terminal_state.update(self.alloc, state.terminal);
 

src/terminal/Terminal.zig

@@ -1625,7 +1625,7 @@ pub const ScrollViewport = union(enum) {
 };
 
 /// Scroll the viewport of the terminal grid.
-pub fn scrollViewport(self: *Terminal, behavior: ScrollViewport) !void {
+pub fn scrollViewport(self: *Terminal, behavior: ScrollViewport) void {
     self.screens.active.scroll(switch (behavior) {
         .top => .{ .top = {} },
         .bottom => .{ .active = {} },

src/terminal/render.zig

@@ -1092,7 +1092,7 @@ test "cursor state out of viewport" {
     try testing.expectEqual(1, state.cursor.viewport.?.y);
 
     // Scroll the viewport
-    try t.scrollViewport(.top);
+    t.scrollViewport(.top);
     try state.update(alloc, &t);
 
     // Set a style on the cursor

src/terminal/search/viewport.zig

@@ -358,7 +358,7 @@ test "history search, no active area" {
     try testing.expect(t.screens.active.pages.pages.first != t.screens.active.pages.pages.last);
     try s.nextSlice("Buzz\r\nFizz");
 
-    try t.scrollViewport(.top);
+    t.scrollViewport(.top);
 
     var search: ViewportSearch = try .init(alloc, "Fizz");
     defer search.deinit();

src/termio/Termio.zig

@@ -641,10 +641,13 @@ pub fn clearScreen(self: *Termio, td: *ThreadData, history: bool) !void {
 }
 
 /// Scroll the viewport
-pub fn scrollViewport(self: *Termio, scroll: terminalpkg.Terminal.ScrollViewport) !void {
+pub fn scrollViewport(
+    self: *Termio,
+    scroll: terminalpkg.Terminal.ScrollViewport,
+) void {
     self.renderer_state.mutex.lock();
     defer self.renderer_state.mutex.unlock();
-    try self.terminal.scrollViewport(scroll);
+    self.terminal.scrollViewport(scroll);
 }
 
 /// Jump the viewport to the prompt.

src/termio/Thread.zig

@@ -321,7 +321,7 @@ fn drainMailbox(
             .resize => |v| self.handleResize(cb, v),
             .size_report => |v| try io.sizeReport(data, v),
             .clear_screen => |v| try io.clearScreen(data, v.history),
-            .scroll_viewport => |v| try io.scrollViewport(v),
+            .scroll_viewport => |v| io.scrollViewport(v),
             .selection_scroll => |v| {
                 if (v) {
                     self.startScrollTimer(cb);

src/termio/stream_handler.zig

@@ -232,7 +232,7 @@ pub const StreamHandler = struct {
             .erase_display_below => self.terminal.eraseDisplay(.below, value),
             .erase_display_above => self.terminal.eraseDisplay(.above, value),
             .erase_display_complete => {
-                try self.terminal.scrollViewport(.{ .bottom = {} });
+                self.terminal.scrollViewport(.{ .bottom = {} });
                 self.terminal.eraseDisplay(.complete, value);
             },
             .erase_display_scrollback => self.terminal.eraseDisplay(.scrollback, value),