Reparenting children tasks is generally broken, and doing so before and after switching between Open and Actionable views will lead to a freeze hang

[nekohayo]

As discovered while reporting #1017:

The video below shows an issue with the completed GTK4 port, using the default dataset, where you can eventually make GTG freeze/hang.

The only relevant line in the terminal is this:

Gtk-CRITICAL **: gtk_widget_set_parent: assertion '_gtk_widget_get_parent (widget) == NULL' failed

GTG-GTK4-reparenting-deselects-sidebar-tag-and-eventually-hangs.webm

This bug is one of the issues I had previously reported in #894 (comment), retranscribed below.

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General parenting bugs (including ones reported before):

• ⚠️ If you deparent a child by dragging (i.e. dragging it between tasks) in a view of Open tasks with many tasks (ex: a hundred), the app freezes (it did that previously, but for a moment yesterday we thought maybe this was due to the infinite loop that happened on startup… well nope, that was not the cause, it was just in addition to the hang!)
• ⚠️ If you drag a child out of a parent, the selected tag in the sidebar resets to "All Tasks" view
• ⚠️ If you drag a standalone task into a parent (turning the task into a child), the selected tag in the sidebar resets to "All Tasks" view
• ⚠️ If you drag a child from one parent to another task (re-parent), it duplicates it in the view
• After reparenting children via drag and drop, doing "Open Parent" twice causes a KeyError traceback, can lead to data file corruption from base_store #1029