fix: backport libghostty resize overflow fix

refs #465

4 files changed, 211 insertions, 2 deletions

diff --git a/vendor/libghostty-vt.patches.md b/vendor/libghostty-vt.patches.md
new file mode 100644
index 00000000..98b7569e
--- /dev/null
+++ b/vendor/libghostty-vt.patches.md
@@ -0,0 +1,40 @@
+# libghostty-vt local patches
+
+This file tracks intentional local changes applied on top of the vendored
+`libghostty-vt` source. Remove a patch only when the vendored source commit
+contains the upstream fix and the listed verification still passes.
+
+## 0001 backport resizeCols cursor subtraction saturation
+
+status: active
+
+patch: `vendor/patches/libghostty-vt/0001-backport-resizecols-cursor-subtraction.patch`
+
+herdr issue: https://github.com/ogulcancelik/herdr/issues/465
+
+upstream discussion: https://github.com/ghostty-org/ghostty/discussions/12905
+
+upstream pr: https://github.com/ghostty-org/ghostty/pull/12907
+
+introduced upstream: `c44afa625`
+
+vendored base: `0f7cd84b880b203c98683e520e84b9db0c5938d8`
+
+local files:
+
+- `vendor/libghostty-vt/src/terminal/PageList.zig`
+- `vendor/libghostty-vt/src/terminal/c/terminal.zig`
+
+reason: shrinking rows and columns in one resize can leave the pre-resize
+cursor row past the new row count. `PageList.resizeCols` then computed rows
+below the cursor with checked unsigned subtraction and aborted in safety builds.
+
+remove when: the vendored source commit contains upstream PR #12907 and the
+local ReleaseSafe resize regression tests pass without this patch.
+
+verification:
+
+```sh
+zig build test-lib-vt -Demit-lib-vt -Doptimize=ReleaseSafe -Dtest-filter="resize shrinks both axes with cursor at bottom"
+zig build test-lib-vt -Demit-lib-vt -Doptimize=ReleaseSafe -Dtest-filter="PageList resize less rows and cols cursor at bottom"
+```
diff --git a/vendor/libghostty-vt/src/terminal/PageList.zig b/vendor/libghostty-vt/src/terminal/PageList.zig
index 8e5cd193..63155962 100644
--- a/vendor/libghostty-vt/src/terminal/PageList.zig
+++ b/vendor/libghostty-vt/src/terminal/PageList.zig
@@ -1059,7 +1059,7 @@ fn resizeCols(
         break :cursor .{
             .tracked_pin = c.pin orelse try self.trackPin(p),
             .untrack = c.pin == null,
-            .remaining_rows = self.rows - c.y - 1,
+            .remaining_rows = self.rows -| (c.y + 1),
             .wrapped_rows = wrapped,
         };
     } else null;
@@ -1192,7 +1192,7 @@ fn resizeCols(
             break :wrapped wrapped;
         };
 
-        const current = self.rows - active_pt.active.y - 1;
+        const current = self.rows -| (active_pt.active.y + 1);
 
         var req_rows = c.remaining_rows;
         req_rows -|= wrapped -| c.wrapped_rows;
@@ -10828,6 +10828,37 @@ test "PageList resize (no reflow) less rows and cols" {
     }
 }
 
+test "PageList resize less rows and cols cursor at bottom" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var s = try init(alloc, 80, 24, 0);
+    defer s.deinit();
+
+    const cursor_pin = try s.trackPin(s.pin(.{ .active = .{
+        .x = 0,
+        .y = s.rows - 1,
+    } }).?);
+    defer s.untrackPin(cursor_pin);
+
+    // Shrink both axes so the original cursor.y is strictly past the new row
+    // count, after resizeWithoutReflow has already reduced self.rows.
+    try s.resize(.{
+        .cols = 79,
+        .rows = 20,
+        .reflow = true,
+        .cursor = .{ .x = 0, .y = 23, .pin = cursor_pin },
+    });
+    try testing.expectEqual(@as(usize, 79), s.cols);
+    try testing.expectEqual(@as(usize, 20), s.rows);
+
+    // remaining_rows saturates to 0, so the cursor lands on the new bottom row.
+    try testing.expectEqual(point.Point{ .active = .{
+        .x = 0,
+        .y = s.rows - 1,
+    } }, s.pointFromPin(.active, cursor_pin.*).?);
+}
+
 test "PageList resize (no reflow) more rows and less cols" {
     const testing = std.testing;
     const alloc = testing.allocator;
diff --git a/vendor/libghostty-vt/src/terminal/c/terminal.zig b/vendor/libghostty-vt/src/terminal/c/terminal.zig
index 5a5db2d6..39959f83 100644
--- a/vendor/libghostty-vt/src/terminal/c/terminal.zig
+++ b/vendor/libghostty-vt/src/terminal/c/terminal.zig
@@ -1005,6 +1005,29 @@ test "resize invalid value" {
     try testing.expectEqual(Result.invalid_value, resize(t, 80, 0, 9, 18));
 }
 
+test "resize shrinks both axes with cursor at bottom" {
+    var t: Terminal = null;
+    try testing.expectEqual(Result.success, new(
+        &lib.alloc.test_allocator,
+        &t,
+        .{
+            .cols = 80,
+            .rows = 24,
+            .max_scrollback = 0,
+        },
+    ));
+    defer free(t);
+
+    // CSI 24;1H parks the cursor on the bottom row.
+    const move = "\x1b[24;1H";
+    vt_write(t, move, move.len);
+
+    // Shrink both axes; pre-resize cursor.y sits past the new bottom row.
+    try testing.expectEqual(Result.success, resize(t, 79, 23, 8, 16));
+    try testing.expectEqual(79, t.?.terminal.cols);
+    try testing.expectEqual(23, t.?.terminal.rows);
+}
+
 test "mode_get and mode_set" {
     var t: Terminal = null;
     try testing.expectEqual(Result.success, new(
diff --git a/vendor/patches/libghostty-vt/0001-backport-resizecols-cursor-subtraction.patch b/vendor/patches/libghostty-vt/0001-backport-resizecols-cursor-subtraction.patch
new file mode 100644
index 00000000..1f8d0f36
--- /dev/null
+++ b/vendor/patches/libghostty-vt/0001-backport-resizecols-cursor-subtraction.patch
@@ -0,0 +1,115 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: herdr maintainers <maintainers@herdr.dev>
+Date: Thu, 4 Jun 2026 00:00:00 +0000
+Subject: [PATCH] backport resizeCols cursor subtraction saturation
+
+Backport ghostty-org/ghostty#12907 onto the vendored libghostty-vt source.
+Shrink-both resize can leave the old cursor row past the new row count after
+resizeWithoutReflow reduces self.rows. Saturating subtraction treats rows below
+that cursor as zero instead of trapping in safety builds.
+
+Herdr issue: https://github.com/ogulcancelik/herdr/issues/465
+Upstream discussion: https://github.com/ghostty-org/ghostty/discussions/12905
+Upstream PR: https://github.com/ghostty-org/ghostty/pull/12907
+Vendored base: 0f7cd84b880b203c98683e520e84b9db0c5938d8
+---
+ vendor/libghostty-vt/src/terminal/PageList.zig    | 33 +++++++++++++++++++++--
+ vendor/libghostty-vt/src/terminal/c/terminal.zig  | 23 ++++++++++++++++
+ 2 files changed, 54 insertions(+), 2 deletions(-)
+
+diff --git a/vendor/libghostty-vt/src/terminal/PageList.zig b/vendor/libghostty-vt/src/terminal/PageList.zig
+index 0000000..0000000 100644
+--- a/vendor/libghostty-vt/src/terminal/PageList.zig
++++ b/vendor/libghostty-vt/src/terminal/PageList.zig
+@@ -1059,7 +1059,7 @@ fn resizeCols(
+         break :cursor .{
+             .tracked_pin = c.pin orelse try self.trackPin(p),
+             .untrack = c.pin == null,
+-            .remaining_rows = self.rows - c.y - 1,
++            .remaining_rows = self.rows -| (c.y + 1),
+             .wrapped_rows = wrapped,
+         };
+     } else null;
+@@ -1192,7 +1192,7 @@ fn resizeCols(
+             break :wrapped wrapped;
+         };
+ 
+-        const current = self.rows - active_pt.active.y - 1;
++        const current = self.rows -| (active_pt.active.y + 1);
+ 
+         var req_rows = c.remaining_rows;
+         req_rows -|= wrapped -| c.wrapped_rows;
+@@ -10828,6 +10828,37 @@ test "PageList resize (no reflow) less rows and cols" {
+     }
+ }
+ 
++test "PageList resize less rows and cols cursor at bottom" {
++    const testing = std.testing;
++    const alloc = testing.allocator;
++
++    var s = try init(alloc, 80, 24, 0);
++    defer s.deinit();
++
++    const cursor_pin = try s.trackPin(s.pin(.{ .active = .{
++        .x = 0,
++        .y = s.rows - 1,
++    } }).?);
++    defer s.untrackPin(cursor_pin);
++
++    // Shrink both axes so the original cursor.y is strictly past the new row
++    // count, after resizeWithoutReflow has already reduced self.rows.
++    try s.resize(.{
++        .cols = 79,
++        .rows = 20,
++        .reflow = true,
++        .cursor = .{ .x = 0, .y = 23, .pin = cursor_pin },
++    });
++    try testing.expectEqual(@as(usize, 79), s.cols);
++    try testing.expectEqual(@as(usize, 20), s.rows);
++
++    // remaining_rows saturates to 0, so the cursor lands on the new bottom row.
++    try testing.expectEqual(point.Point{ .active = .{
++        .x = 0,
++        .y = s.rows - 1,
++    } }, s.pointFromPin(.active, cursor_pin.*).?);
++}
++
+ test "PageList resize (no reflow) more rows and less cols" {
+     const testing = std.testing;
+     const alloc = testing.allocator;
+diff --git a/vendor/libghostty-vt/src/terminal/c/terminal.zig b/vendor/libghostty-vt/src/terminal/c/terminal.zig
+index 0000000..0000000 100644
+--- a/vendor/libghostty-vt/src/terminal/c/terminal.zig
++++ b/vendor/libghostty-vt/src/terminal/c/terminal.zig
+@@ -1005,6 +1005,29 @@ test "resize invalid value" {
+     try testing.expectEqual(Result.invalid_value, resize(t, 80, 0, 9, 18));
+ }
+ 
++test "resize shrinks both axes with cursor at bottom" {
++    var t: Terminal = null;
++    try testing.expectEqual(Result.success, new(
++        &lib.alloc.test_allocator,
++        &t,
++        .{
++            .cols = 80,
++            .rows = 24,
++            .max_scrollback = 0,
++        },
++    ));
++    defer free(t);
++
++    // CSI 24;1H parks the cursor on the bottom row.
++    const move = "\x1b[24;1H";
++    vt_write(t, move, move.len);
++
++    // Shrink both axes; pre-resize cursor.y sits past the new bottom row.
++    try testing.expectEqual(Result.success, resize(t, 79, 23, 8, 16));
++    try testing.expectEqual(79, t.?.terminal.cols);
++    try testing.expectEqual(23, t.?.terminal.rows);
++}
++
+ test "mode_get and mode_set" {
+     var t: Terminal = null;
+     try testing.expectEqual(Result.success, new(
+-- 
+2.49.0