Why layer number 3 becomes invisible?

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the problem seems to be that Construct doesn't like to change the visibility of non-existent layers. Your loops go up to 20, but you only have 4 layers. if you change the loops to "0 to 3" then it works the way you expect.

I couldn't find a way to detect how many layers there are at runtime. You could create a global variable "LayerCount" if you are planning on adding or removing layers - just remember to update the global variable...

Ok I understand.

Maybe it's something that can be solved, maybe a bug.

Thanks for the help.

You can simply add another event to the function, after the loop - Set "Layer 3" visible

Another solution would be renaming all layers that you want to change visibility like this: L1, L2, L3, L4...

And change your code: For ""=1 to 20 : Set layer "L"&loopindex invisible