fix-deviant(renderer): don't over-allocate Metal buffers by sizeof(T)

Intentional deviation from Ghostty 1.3.1 (DEVIANT_FIXES.md row 34).
When a Metal buffer grows, Zig's renderer/metal/buffer.zig:89 and :146
compute the new size as twice the required *byte* count and then
multiply by @sizeOf(T) again, so large cell buffers allocate
sizeof(T) times more GPU memory than intended. The Rust Metal backend
(since bab49ee) allocates exactly twice the required bytes; this commit
factors the computation into metal_layout::grown_buffer_bytes, adds a
regression test and records the deviation.

Test Plan:
- cargo test -p ghostty-renderer
  grown_buffer_bytes_is_double_the_required_bytes_not_scaled_by_size_of_t
- just check-macos

crates/ghostty-renderer/src/metal/buffer.rs

@@ -9,7 +9,7 @@ use objc2_metal::{MTLBuffer, MTLDevice as _, MTLResourceOptions};
 use super::{Device, Error};
 use crate::{
     graphics_api::{ApiError, GpuBuffer},
-    metal_layout::{ResourceOptions, StorageMode},
+    metal_layout::{ResourceOptions, StorageMode, grown_buffer_bytes},
 };
 
 /// Options for initializing a buffer.
@@ -124,8 +124,9 @@ impl<T: Pod> Buffer<T> {
             // require (the old buffer is released when it is replaced).
             //
             // NOTE: Zig multiplies this size by `@sizeOf(T)` a second time,
-            // over-allocating; we allocate exactly double.
+            // over-allocating; we allocate exactly double
-            let size = req_bytes * 2;
+            // (see `grown_buffer_bytes`).
+            let size = grown_buffer_bytes(req_bytes);
             self.buffer = self
                 .opts
                 .device

crates/ghostty-renderer/src/metal_layout.rs

@@ -478,6 +478,19 @@ pub const PIPELINE_DESCS: [PipelineDesc; 5] = [
     },
 ];
 
+/// Size in bytes of the replacement Metal buffer when `req_bytes` bytes no
+/// longer fit (`metal/buffer.zig` `sync`/`syncFromArrayLists`: "enough to
+/// hold double what we require").
+///
+/// Upstream bug fixed: Zig computes `size = req_bytes * 2` and then passes
+/// `size * @sizeOf(T)` to `newBufferWithLength:options:`, multiplying by the
+/// element size a second time (`sizeof(T)` times too much GPU memory). The
+/// length is already in bytes, so this is exactly double.
+#[must_use]
+pub const fn grown_buffer_bytes(req_bytes: usize) -> usize {
+    req_bytes * 2
+}
+
 #[cfg(test)]
 mod tests {
     use core::mem::{offset_of, size_of};
@@ -485,6 +498,16 @@ mod tests {
     use super::*;
     use crate::shader_types::{Bools, PaddingExtend};
 
+    #[test]
+    fn grown_buffer_bytes_is_double_the_required_bytes_not_scaled_by_size_of_t() {
+        // 100 vertices of a 32-byte type: 3200 bytes required, 6400 allocated
+        // (Zig would ask Metal for 6400 * 32 bytes).
+        let req_bytes = 100 * size_of::<CellText>();
+        assert_eq!(grown_buffer_bytes(req_bytes), req_bytes * 2);
+        assert_eq!(grown_buffer_bytes(0), 0);
+        assert_eq!(grown_buffer_bytes(1), 2);
+    }
+
     #[test]
     fn resource_options_bits() {
         // MTLResourceCPUCacheModeWriteCombined | MTLResourceStorageModeShared

docs/rewrite/DEVIANT_FIXES.md

@@ -59,6 +59,7 @@ when this file was written.
 | 31 | GTK: `+new-window` D-Bus arguments | `apprt/gtk/ipc/new_window.zig:49-53` (`Bytes.new(ptr, len + 1)` + `Variant.newFromBytes(..., trusted = true)`) | Arguments are passed as trusted GVariant strings without validation; invalid UTF-8 or an interior NUL makes GIO abort the process while serializing (input-triggered crash). | `arguments_variant` validates first and returns `Error::IpcFailed` (message on stderr, no D-Bus call); valid arguments are byte-identical to Zig. | `65dd9d5` fix-deviant(gtk) | `ipc::new_window::tests::invalid_dbus_strings_are_rejected_without_abort` (`ghostty-apprt-gtk`) |
 | 32 | OS: URL opener child reaping | `os/open.zig` (`try collectOutput(...)` before `wait()`) | When collecting the opener's output fails (e.g. more than 50 KiB on a stream), the error returns before `wait()`, leaving the child as a zombie. | The child is always reaped; the collection error is still logged. | `4a55abf` fix-deviant(os) | `ghostty-os` open tests |
 | 33 | OS: maximum-length `NOTIFY_SOCKET` | `os/systemd.zig:100-116` (`socket_address.path.len < socket_path.len`, then `path[socket_path.len] = 0`) | A 108-byte path (the size of `sockaddr_un.path`) passes the length check, and the terminating NUL is written at `path[108]`, one byte past the array (out-of-bounds write). Separate from row 13 (the abstract-socket `@` handling). | The address is built with rustix's `SocketAddrUnix::new`, which takes the 108 bytes without a terminator (valid for Linux `AF_UNIX`) and rejects longer paths; no out-of-bounds write. Verified in code. | f8c5ce0 (`git log -S` on `ghostty-os/src/systemd.rs`: the only commit touching it, besides formatting/macOS changes) | `notify_maximum_length_socket_path_is_rejected_safely` (`ghostty-os/src/systemd.rs`) |
+| 34 | Renderer (Metal): buffer reallocation size | `renderer/metal/buffer.zig:89` (`Buffer.sync`) and `:146` (`syncFromArrayLists`) | `size = req_bytes * 2` is already in bytes, then `size * @sizeOf(T)` is passed to `newBufferWithLength:options:`, so the new buffer is `sizeof(T)` times larger than intended (GPU-memory over-allocation). | The new buffer is exactly `2 * req_bytes` (`metal_layout::grown_buffer_bytes`); the element count is `2 * total_len`. | bab49ee (`git log -S"over-allocating"` on `ghostty-renderer/src/metal/buffer.rs`; the file's first commit) | `metal_layout::tests::grown_buffer_bytes_is_double_the_required_bytes_not_scaled_by_size_of_t` (runs on any host; the Metal buffer itself is compile-checked only) |
 
 ## Upstream quirks deliberately kept (not fixed)