Right now it's super non-obvious that you can get the folder size by setting the cursor over a folder icon and waiting a second. My guess is that we are waiting for that second to start calculating to avoid unnecessarily thrashing the disk if someone is just quickly hovering, but it makes the whole feature unapparent.
To fix this, I recommend we serve up a couple of tweaks plus "white lies" to make it more obvious that there's size information available here.
1. Show a spinner when actually calculating the numbers. Right now we display, "calculating..." in place of the values, but a spinner, if possible, would be simpler and more clear indication that an active process is occurring.
2. Show that spinner during the waiting period as well. If we show the spinner any time someone is hovering a folder icon, it makes it more clear that if they wait for that spinner to stop, they're going to get more information.
3. (If possible) Cache the folder size information and show it immediately the next time someone hovers the folder's icon, then after the second wait is up, recalculate and update the label. This avoids showing the spinner so often and makes the file browser feel even faster.
