Fix IconView blank after cut/paste between parent-child folders

[Dipanshusinghh]

Fixes #2801

Added child_notify_frozen boolean flag in IconView.vala and
AbstractTreeView.vala to ensure freeze_child_notify() and
thaw_child_notify() calls are always balanced, preventing IconView
from going permanently blank after cut/paste operations.

[jeremypw]

This does seem to fix the issue for me, I am afraid. According to the documentation I have found there is no need to pair freeze_child_notify and thaw_child_notify calls (unlike object.freeze_notify and object.thaw_notify). When testing, make sure that you move a file between a parent folder and a child folder and then undo.

[Dipanshusinghh]

Understod ! i will update this by today☺️

[jeremypw]

You are right that the calls to the child_notify functions are unbalanced though. This whole area of the code is rather ugly and is a hack to improve the performance of IconView. Whether it is worth spending a lot of time making it more elegant is questionable as it will go away once we switch to Gtk4 dynamic views (unless it causes an issue).

[Dipanshusinghh]

Thnks.
for the clarification since the documentation says pairing is not strictly necessary should I revert the boolean flag approach and instead focus on fixing the root cause of why thaw_child_notify() is being called without a prior freeze_child_notify()? Or would you prefer a different approach? Happy to update as per your suggestion

[jeremypw]

@Dipanshusinghh You need to identify and fix the root cause of #2801 to complete this PR. If you want to improve the code for other reasons it would be better to open a separate PR for that.

The unpaired calls do not seem to be causing any actual issue do they?

Note that the issue only occurs with parent - child source and destination folders so the cause must have something to do with that. The cause could be quite obscure so I would try to determine (for example) whether the model for the view is being filled and not displayed by adding debugging messages or otherwise. Or maybe its a race between the view being unthawed and being drawn.