[bo2themax]

Found this issue when testing some search features; follow up for #12907.

You can either reproduce using the PoC below with libghostty-vt or run Ghostty debug on macOS and follow these steps:

1. Open Ghostty and start search 0.
2. Press cmd+= to increase font size.
3. You should see a panic crash.

AI Disclosure

As the commit suggests, Claude implemented the fix, the unit test, and PoC file.

I reviewed it(seems reasonable to me, but I’m not a Zig professional) and tested it myself.

// PoC: resize panic when shrinking both axes with the cursor near the top
// of a fully-populated screen.
//
// Build (with libghostty-vt headers + dylib on the standard search paths):
//   zig run poc.zig -lghostty-vt
//
// Or point at a local build:
//   zig run poc.zig -I <prefix>/include -L <prefix>/lib -lghostty-vt
//
// At runtime the dylib must be discoverable (DYLD_LIBRARY_PATH on macOS,
// LD_LIBRARY_PATH on Linux, or an rpath baked in at link time).
//
// Without the fix, this aborts with
//   reached unreachable code   (assert in PageList.Pin.pageIterator)
// at _terminal.PageList.resizeCols on a debug/safe build. On release it
// silently iterates an empty (reversed) range.

const std = @import("std");
const c = @cImport({
    @cInclude("ghostty/vt.h");
});

pub fn main() !void {
    var term: c.GhosttyTerminal = null;
    const opts: c.GhosttyTerminalOptions = .{
        .cols = 80,
        .rows = 24,
        .max_scrollback = 1000,
    };
    if (c.ghostty_terminal_new(null, &term, opts) != c.GHOSTTY_SUCCESS) {
        return error.InitFailed;
    }
    defer c.ghostty_terminal_free(term);

    // Fill every one of the 24 active rows with non-blank content. This is
    // what makes the bug reachable: when rows shrink, resizeWithoutReflow
    // can only trim *blank* trailing rows, so non-blank rows are instead
    // pushed up into scrollback and the active-area top moves down.
    {
        var buf: [256]u8 = undefined;
        var i: usize = 0;
        while (i < 24) : (i += 1) {
            // "X" on each row; CR+LF between rows but not after the last so
            // we don't scroll the top row away.
            const line = if (i + 1 < 24)
                std.fmt.bufPrint(&buf, "X\r\n", .{}) catch unreachable
            else
                std.fmt.bufPrint(&buf, "X", .{}) catch unreachable;
            c.ghostty_terminal_vt_write(term, line.ptr, line.len);
        }
    }

    // CSI 1;1H -> park the cursor on the TOP row (1-based). The active area is
    // anchored to the bottom, so once we shrink rows this row falls above the
    // new active-area top, i.e. into scrollback.
    const move = "\x1b[1;1H";
    c.ghostty_terminal_vt_write(term, move.ptr, move.len);

    // Shrink both axes. Columns must shrink to take resize()'s .lt branch,
    // which runs the row shrink first and then resizeCols with the original
    // (now out-of-active-area) cursor pin. Panics in
    // _terminal.PageList.resizeCols.
    _ = c.ghostty_terminal_resize(term, 79, 20, 8, 16);

    std.debug.print("survived resize (fix is present)\n", .{});
}