applets/taskmanager: Ask for a thumbnail when window is ready for painting

Vlad Zahorodniirequested to merge
work/zzag/thumbnail-ready-for-painting into master

Screencast requests will be rejected until the window is ready for painting.

This change makes sure that the task manager doesn't ask for a thumbnail too early. It fixes task manager tooltips having blank thumbnails sometimes.

Screenshot_20260819_163911

Edited by Vlad Zahorodnii

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