font/sprite: rect must be smaller than canvas dims

src/font/sprite/canvas.zig

@@ -424,9 +424,6 @@ 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),
@@ -436,6 +433,10 @@ const PixmanImpl = struct {
             },
         };
 
+        assert(boxes[0].x1 >= 0);
+        assert(boxes[0].y1 >= 0);
+        assert(boxes[0].x2 <= @as(i32, @intCast(self.image.getWidth())));
+        assert(boxes[0].y2 <= @as(i32, @intCast(self.image.getHeight())));
         self.image.fillBoxes(.src, color.pixmanColor(), boxes) catch {};
     }
 

src/font/sprite/underline.zig

@@ -80,13 +80,14 @@ 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 -| self.thickness));
+        const y = bottom -| self.thickness;
+        const max_height = self.height - y;
 
         canvas.rect(.{
             .x = 0,
-            .y = y,
+            .y = @intCast(y),
             .width = self.width,
-            .height = self.thickness,
+            .height = @min(self.thickness, max_height),
         }, .on);
     }