writing [ or ] into a sign breaks the form for some users

the exact effect varies with client behavior and the particular text contents.
(characters erased, disappering formspec elements, loss of text)

api.lua

@@ -623,11 +623,10 @@ end
 
 function signs_lib.rightclick_sign(pos, node, player, itemstack, pointed_thing)
 
-	local playername = player:get_player_name()
+	if not signs_lib.can_modify(pos, player) then return end
-	if not minetest.check_player_privs(playername, {signslib_edit = true}) then return end
 
 	player:get_meta():set_string("signslib:pos", minetest.pos_to_string(pos))
-	minetest.show_formspec(playername, "signs_lib:sign", get_sign_formspec(pos, node.name))
+	minetest.show_formspec(player:get_player_name(), "signs_lib:sign", get_sign_formspec(pos, node.name))
 end
 
 function signs_lib.destruct_sign(pos)
@@ -1152,7 +1151,7 @@ function get_sign_formspec(pos, nodename)
 
 	local formspec = {
 		"size[6,4]",
-		"textarea[0,-0.3;6.5,3;text;;" .. txt .. "]",
+		"textarea[0,-0.3;6.5,3;text;;" .. minetest.formspec_escape(txt) .. "]",
 		"background[-0.5,-0.5;7,5;signs_lib_sign_bg.jpg]",
 		"button_exit[2,3.4;2,1;ok;" .. S("Write") .. "]"
 	}
@@ -1196,14 +1195,15 @@ minetest.register_on_player_receive_fields(function(player, formname, fields)
 			meta:set_int("widefont", 1)
 			change = true
 		end
+
 		if change then
 			minetest.log("action", S("@1 flipped the wide-font switch to \"@2\" at @3",
 				(playername or ""),
 				(fields.on and "off" or "on"),
 				minetest.pos_to_string(pos)
 			))
-			minetest.show_formspec(playername, "signs_lib:sign", get_sign_formspec(pos, node.name))
 			signs_lib.update_sign(pos, fields)
+			minetest.show_formspec(playername, "signs_lib:sign", get_sign_formspec(pos, node.name))
 		end
 	end
 end)