I saw this thing yesterday on my window curtain, and I've been wondering what this is. It looks like a dense network of spider web, and the spider inside it is visible. I thought the spider was dead, but I saw the spider making very small movements inside it. So, that made me more curious to know what it is.
From the look of the spider outline, it looks to have short legs and is dark. A yellow sac spider is quite light-coloured and has long legs. Yellow sacs and clubionids will sometimes make themselves a sac for resting or eating or molting or laying eggs. Yours is probably a jumping spider, and here is a youtube timelapse video of a jumping spider laying eggs while inside its own protective sac: