frame pacing fix

cmd/mxl-player/frame_pacing_test.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"testing"
+	"time"
+)
+
+func TestRemainingFrameTime(t *testing.T) {
+	start := time.Date(2026, time.September, 2, 0, 0, 0, 0, time.UTC)
+	interval := 16 * time.Millisecond
+	tests := []struct {
+		name    string
+		elapsed time.Duration
+		want    time.Duration
+	}{
+		{name: "no work", want: interval},
+		{name: "partial budget", elapsed: 5 * time.Millisecond, want: 11 * time.Millisecond},
+		{name: "exact budget", elapsed: interval},
+		{name: "over budget", elapsed: 20 * time.Millisecond},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got := remainingFrameTime(start, start.Add(test.elapsed), interval)
+			if got != test.want {
+				t.Fatalf("remainingFrameTime() = %v, want %v", got, test.want)
+			}
+		})
+	}
+}