Pallette positioned away from parent button after resize

[quozl]

When the screen dimensions are increased, any pallette windows that are outside the original screen dimensions are instead shown on the top left of the screen.

Reproducer:

• start Sugar in a VM with screen dimensions of 1024x768, and start Terminal activity,
• move the mouse cursor into the stop button and observe the pallette window is correctly positioned under the stop button,
• move the mouse cursor back into the terminal window,
• resize the screen to 1280x720, by typing command xrandr --output Virtual-0 --mode 1280x720,
• move the mouse cursor into the stop button and observe the pallette window is incorrectly positioned to top left of screen,

Also occurs with tool icons in Home View, the activity ring.

Observed on Ubuntu 17.10 with Sugar 0.112 and some earlier versions.

There are no unusual messages in shell.log or org.laptop.Terminal-1.log.

Cause may be Invoker in pallettewindow.py where screen width is cached by __init__.