Live Previews of Line Break Changes
When you drag a side handle of a text frame to change its width, the frame side moves but the text stays where it was, at its original width and with its original line breaks. It's not until you release the mouse button that you see the text rewrap to the new frame width.
If you want, you can monitor how the line breaks change in response to your resizing as you drag in real-time. The trick is hold your mouse button down on the side handle for a couple seconds before you start to drag. You'll see the double-headed arrow cursor flash to a single large arrowhead briefly and then change back to the double-headed arrow.
Wait for that flash, and now as you drag, you'll see the lines recompose themselves to keep up with the ever-changing width of the text frame. It's perfect for those times when the reason you're adjusting a text frame's width in the first place is to tweak the line breaks!