config: expose scrollback line limit

src/config/Config.zig

@@ -95,6 +95,10 @@ pub const compatibility = std.StaticStringMap(
     // Ghostty 1.3 rename the "window" option to "new-window".
     // See: https://github.com/ghostty-org/ghostty/pull/9764
     .{ "macos-dock-drop-behavior", compatMacOSDockDropBehavior },
+
+    // Ghostty 1.4 renamed `scrollback-limit` to `scrollback-limit-bytes`
+    // when `scrollback-limit-lines` was added so the units are explicit.
+    .{ "scrollback-limit", cli.compatibilityRenamed(Config, "scrollback-limit-bytes") },
 });
 
 /// Set Ghostty's graphical user interface language to a language other than the
@@ -1375,11 +1379,32 @@ input: RepeatableReadableIO = .{},
 ///
 /// This size is per terminal surface, not for the entire application.
 ///
-/// It is not currently possible to set an unlimited scrollback buffer.
-/// This is a future planned feature.
+/// A separate maximum can be set with `scrollback-limit-lines`; if both limits
+/// are set, then the first one reached will determine when scrollback is
+/// removed.
+///
+/// It is not currently possible to set an unlimited byte limit.
 ///
 /// This can be changed at runtime but will only affect new terminal surfaces.
-@"scrollback-limit": usize = 50_000_000, // 50MB
+@"scrollback-limit-bytes": usize = 50_000_000, // 50MB
+
+/// The maximum number of lines of scrollback to retain. This excludes
+/// the active screen. Soft-wrapped lines count as multiple lines.
+///
+/// This limit is an estimate. Internally, Ghostty will only trim lines
+/// up to the minimum allocation unit that is used internally (called a
+/// "page"). The size of a page depends on how many styles, graphemes, etc.
+/// take up the screen. In practice, this can be anywhere from a handful to
+/// a couple hundred lines. Importantly, memory is capped either way.
+/// This means that the actual limited lines will likely be slightly
+/// higher in practice.
+///
+/// If this is unset, then no line limit will be applied. A separate maximum
+/// can be set with `scrollback-limit-bytes`; if both limits are set, then the
+/// first one reached will determine when scrollback is removed.
+///
+/// This can be changed at runtime but will only affect new terminal surfaces.
+@"scrollback-limit-lines": ?usize = null,
 
 /// Whether to compress scrollback pages while the terminal is idle.
 ///
@@ -10825,6 +10850,55 @@ test "theme specifying light/dark sets theme usage in conditional state" {
     }
 }
 
+test "scrollback limits" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var cfg = try Config.default(alloc);
+    defer cfg.deinit();
+    var it: TestIterator = .{ .data = &.{
+        "--scrollback-limit-bytes=1234",
+        "--scrollback-limit-lines=567",
+    } };
+    try cfg.loadIter(alloc, &it);
+
+    try testing.expectEqual(
+        @as(usize, 1234),
+        cfg.@"scrollback-limit-bytes",
+    );
+    try testing.expectEqual(
+        @as(?usize, 567),
+        cfg.@"scrollback-limit-lines",
+    );
+
+    var unset_it: TestIterator = .{ .data = &.{
+        "--scrollback-limit-lines=",
+    } };
+    try cfg.loadIter(alloc, &unset_it);
+
+    try testing.expectEqual(
+        @as(?usize, null),
+        cfg.@"scrollback-limit-lines",
+    );
+}
+
+test "compatibility: scrollback-limit renamed to bytes" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var cfg = try Config.default(alloc);
+    defer cfg.deinit();
+    var it: TestIterator = .{ .data = &.{
+        "--scrollback-limit=1234",
+    } };
+    try cfg.loadIter(alloc, &it);
+
+    try testing.expectEqual(
+        @as(usize, 1234),
+        cfg.@"scrollback-limit-bytes",
+    );
+}
+
 test "compatibility: gtk-single-instance desktop" {
     const testing = std.testing;
     const alloc = testing.allocator;

src/termio/Termio.zig

@@ -246,7 +246,8 @@ pub fn init(self: *Termio, alloc: Allocator, opts: termio.Options) !void {
         break :opts .{
             .cols = grid_size.columns,
             .rows = grid_size.rows,
-            .max_scrollback_bytes = opts.full_config.@"scrollback-limit",
+            .max_scrollback_bytes = opts.full_config.@"scrollback-limit-bytes",
+            .max_scrollback_lines = opts.full_config.@"scrollback-limit-lines",
             .default_modes = default_modes,
             .default_cursor_style = opts.config.cursor_style,
             .default_cursor_blink = opts.config.cursor_blink,