Created attachment 276812 [details] HTML file demonstrating issue This is a bit hard to explain, but I've attached a reproducible reduction. The attached HTML file creates a grid using flex box. In response to a Window "resize" event, the grid is resized to fill the entire window. Under Safari Technology Preview, the grid becomes stuck after the initial resize. Steps to Reproduce: 1) Load the HTML in a web browser 2) Resize the window to be larger 3) The grid will snap to the new window size 4) Resize the window to be smaller Expected Result: In Safari 9, Chrome, and Firefox, the grid should continue to resize and fill the window on both the horizontal and vertical axis. Actual Result: In Safari Technology Preview and WebKit nightlies, each row in the grid becomes "stuck" after the initial resize, and won't shorten. The grid continues to work in the horizontal axis.