123mixin.java

  // This injects into the addMessage method of ChatHud, which handles adding messages to the chat.
    @Inject(method = "addMessage(Lnet/minecraft/text/Text;Lnet/minecraft/network/message/MessageSignatureData;Lnet/minecraft/client/gui/hud/MessageIndicator;)V", at = @At("HEAD"))
    private void onAddMessage(Text message, MessageSignatureData signatureData, MessageIndicator indicator, CallbackInfo ci) {
        if (message instanceof MutableText mutableText) {
            // Create a new style with a click event to copy text and a hover event with instructions.
            Style style = mutableText.getStyle().withClickEvent(
                    new ClickEvent(ClickEvent.Action.COPY_TO_CLIPBOARD, mutableText.getString())
            ).withHoverEvent(
                    new HoverEvent(HoverEvent.Action.SHOW_TEXT, Text.literal("Click to copy this message"))
            );
            // Apply the new style to the text.
            mutableText.setStyle(style);
        }
    }