font/sprite: test for thick underline, assertion on x/y for rect

src/font/sprite/canvas.zig

@@ -424,6 +424,9 @@ const PixmanImpl = struct {
     /// Draw and fill a rectangle. This is the main primitive for drawing
     /// lines as well (which are just generally skinny rectangles...)
     pub fn rect(self: *Canvas, v: Rect, color: Color) void {
+        assert(v.x >= 0);
+        assert(v.y >= 0);
+
         const boxes = &[_]pixman.Box32{
             .{
                 .x1 = @intCast(v.x),

src/font/sprite/underline.zig

@@ -80,7 +80,7 @@ const Draw = struct {
         // Ensure we never overflow out of bounds on the canvas
         const y_max = self.height -| 1;
         const bottom = @min(self.pos + self.thickness, y_max);
-        const y = @as(i32, @intCast(bottom)) - @as(i32, @intCast(self.thickness));
+        const y = @as(i32, @intCast(bottom -| self.thickness));
 
         canvas.rect(.{
             .x = 0,
@@ -224,6 +224,26 @@ test "single" {
     );
 }
 
+test "single large thickness" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var atlas_greyscale = try font.Atlas.init(alloc, 512, .greyscale);
+    defer atlas_greyscale.deinit(alloc);
+
+    // unrealistic thickness but used to cause a crash
+    // https://github.com/mitchellh/ghostty/pull/1548
+    _ = try renderGlyph(
+        alloc,
+        &atlas_greyscale,
+        .underline,
+        36,
+        18,
+        9,
+        200,
+    );
+}
+
 test "curly" {
     const testing = std.testing;
     const alloc = testing.allocator;