SDL HitTests will make window invalid.

[tDwtp]

I tried to use HitTests (in lua 5.2 with SDL 2.0.7).
After some mouse movement inside the window, it suddenly turns invalid.

I made a minimal example here:

local sdl = require "sdl"

do -- initialization
	local ok, msg = sdl.init {sdl.flags.Everything}
	if not ok then
		msg = "sdl: " .. tostring (msg)
		error (msg)
	end
end

-- window creation
local window = sdl.createWindow {
	title  = "HitTest Bug",
	width  = 240,
	height = 240,
	flags  = {
		sdl.window.Resizable
	}
}

do -- hittest
	local Draggable   = sdl.hitTestResult.Draggable
	
	local function hittest (window, area)
		print (window)
		print (string.format ("area: %dx%d", area.x, area.y))
		return Draggable
	end
	
	window:setHitTest (hittest)
end

-- main loop
local running = true
while running do
	-- event "sdl.pollEvent"
	for event in sdl.pollEvent ( ) do
		if event.type == sdl.event.Quit then
			running = false
		end
	end
	
	-- error display
	local err = sdl.getError ( )
	if err and #err > 0 then
		print(err)
		sdl.clearError ( )
		running = false
	end
end

expected output:

[...]
window "HitTest Bug": size 240x240
area: 231x204
window "HitTest Bug": size 240x240
area: 230x204
window "HitTest Bug": size 240x240
area: 229x204
window "HitTest Bug": size 240x240
area: 229x203
Invalid window

[tDwtp]

I think I found the error: It occurs as soon as the garbagecollector runs.
edit 1: adding collectgarbage("collect") directly after the polling loop makes it even more visible.

I assume it is due to the flag mustdelete which is set to 1 by commonPushUserdata. However that window is still in use!

I am not quite confident about my assumption here. I guess mustdelete is used to determin, whether the underlying object must be freed, no the CommonUserdata per se.

edit 1: I recompiled with my fix, but it did not work?
edit 2: Changed the fix; its working now. If I did not undersand the mustdelete-flag correctly, it may cause memory leaks!