Even if it was implemented perfectly, you could work around that using timing side channels. For example, multiply the value (e.g. window width) by some huge n

Even if it was implemented perfectly, you could work around that using timing side channels. For example, multiply the value (e.g. window width) by some huge number, perform a slow operation in a loop that many times, and finally clear a flag. Meanwhile another thread is filling an array one by one until the flag gets cleared. The last non-tainted index in the array indicates your approximate window width.