The MoarSigns GUI cancels the "Place" action of SignPicture, a client-side mod which lets you construct signs with metadata that you and other users of the mod see rendered as pictures rather than a sign with text. SignPicture is used to create elaborate street signs, billboards, wall paintings, and all sorts of cool stuff.

To be compatible with SignPicture, I recommend any of the following:

• A client-side button-togglable option to (temporarily) disable the MoarSign GUI from hijacking that of the vanilla sign (or all signs, whichever approach is easier).
• A client-side configuration option to more permanently disable the MoarSign GUI from hijacking that of the vanilla sign only. It should be possible to change this setting and relaunch Minecraft with the new setting taking effect.
• Dynamic code that prevents the MoarSign GUI from hijacking that of the vanilla sign only when SignPicture is present on the client.
• Talking to the devs of SignPicture (I believe they are Japanese, so there could be a language issue with this suggestion) and find out a way to have the mods communicate when SignPicture is being invoked, and selectively disable the MoarSign GUI while placing a sign that is to be written by SignPicture.

SignPicture is:
Forge: https://minecraft.curseforge.com/projects/signpicture
GitHub: https://github.com/Team-Fruit/SignPicture

Please add compatibility for SignPicture, it is my number 1 favorite mod but MoarSigns looks amazing also. I want both! :D

I don't hijack the vanilla sign gui, the items are my own and I open my own gui from them.
A compat for this looks like it's needed on their side.
I've kept the block very similar from the outside for better compat with mods that interacts with the text on the signs.