I think this is a Weston bug, because there are no global coords that the clients would be using, and this behaviour seems to be similar through multiple toolkits that support it. Trying to resize a window from two corners or two sides at the same time (using weston multiseat) causes the window to jump around the screen erratically, until it eventually jumps offscreen.
This is now disabled: commit 7b83ae433f9f373e211020b92e4a2877b413d8d7 Author: Kristian Høgsberg <krh@bitplanet.net> Date: Tue Apr 29 14:50:25 2014 -0700 desktop-shell: Reject resize if we're already grabbed This rejects resizing a window that's currently being moved. This can be done using a touch screen and a pointer or just two seats.
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