PaperMC/paper-server/nms-patches/net/minecraft/world/level/block/BlockSign.patch

--- a/net/minecraft/world/level/block/BlockSign.java
+++ b/net/minecraft/world/level/block/BlockSign.java
@@ -115,7 +115,7 @@
                 } else if (flag2) {
                     return EnumInteractionResult.SUCCESS;
                 } else if (!this.otherPlayerIsEditingSign(entityhuman, tileentitysign) && entityhuman.mayBuild() && this.hasEditableText(entityhuman, tileentitysign, flag1)) {
-                    this.openTextEdit(entityhuman, tileentitysign, flag1);
+                    this.openTextEdit(entityhuman, tileentitysign, flag1, org.bukkit.event.player.PlayerSignOpenEvent.Cause.INTERACT); // CraftBukkit
                     return EnumInteractionResult.SUCCESS;
                 } else {
                     return EnumInteractionResult.PASS;
@@ -164,6 +164,15 @@
     }
 
     public void openTextEdit(EntityHuman entityhuman, TileEntitySign tileentitysign, boolean flag) {
+        // Craftbukkit start
+        openTextEdit(entityhuman, tileentitysign, flag, org.bukkit.event.player.PlayerSignOpenEvent.Cause.UNKNOWN);
+    }
+
+    public void openTextEdit(EntityHuman entityhuman, TileEntitySign tileentitysign, boolean flag, org.bukkit.event.player.PlayerSignOpenEvent.Cause cause) {
+        if (!org.bukkit.craftbukkit.event.CraftEventFactory.callPlayerSignOpenEvent(entityhuman, tileentitysign, flag, cause)) {
+            return;
+        }
+        // Craftbukkit end
         tileentitysign.setAllowedPlayerEditor(entityhuman.getUUID());
         entityhuman.openTextEdit(tileentitysign, flag);
     }