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quic-go defaults the `InitialPacketSize` to 1280 bytes. This is used for the size of the payload plus UDP header. When quic-go creates its initial handshake packet, it pads it to this size as an optimization for the anti-amplification limit (which does not really apply to Caddy's server connections). Tailscale uses an MTU of 1280 (the minimum IPv6 MTU) because it is a tunnel operating in unknown environments, possibly inside other tunnels. When wrapped in an IP header (either v4 or v6), the 1280 byte QUIC packet constructed by quic-go exceeds the Tailscale MTU and gets dropped. This makes it impossible to respond to an HTTP3 connection attempt over Tailscale when using the default `InitialPacketSize`. We explicitly set the `InitialPacketSize` to 1200 (its minimum) to support HTTP3 connections over Tailscale and other low MTU connections. Once the connection is established, quic-go can perform MTU discovery to increase the packet size to the maximum supported by the connection, so any throughput loss due to the lowered `InitialPacketSize` is short-lived.