Only return valid task item size if task manager has been resized to fit panel
Previously the task manager would briefly have null width and height, before being resized to fit the panel. However, the "preferredMaxWidth" for tasks would still be positive, as it adds margins to it, and tasks would then assume a positive width. When the proper values are set, this casuses an extra resize animation on startup.
TO TEST: restart plasmashell. Did you see task manager moving as soon as the panel appears? it should happen without this patch, and it should not happen with.
Edited by Ilya Bizyaev