Fails to initiliase main GUI window (clutter assertion)

I checked the Octave bug tracker and here and cannot see an existing bug/issue and I believe this to be an flatpak issue.

When I start the GUI version, an icon gets added to the dock (and if I right click that it shows that there is a window, but that window is empty/invisible). In the system log I can see one line which seem relevant:

```
Aug 04 19:14:18 gnome-shell[3470]: clutter_actor_insert_child_at_index: assertion 'child->priv->parent == NULL' failed
```

Not sure when this started to happen (I have not used Octave in a while) but it used to work for me. I strongly suspect that this might be the updated Gnome runtime thing...
If I start Octave flatpack from the terminal (without the `--gui` flag) I get working Octave CLI.

Octave details:

```
flatpak info org.octave.Octave

GNU Octave - Scientific Programming Language

ID: org.octave.Octave
Ref: app/org.octave.Octave/x86_64/stable
Arch: x86_64
Branch: stable
Version: 11.3.0
License: GPL-3.0+
Origin: flathub
Collection: org.flathub.Stable
Installation: system
Installed: 550.7 MB
Runtime: org.kde.Sdk/x86_64/6.10
Sdk: org.kde.Sdk/x86_64/6.10

Commit: a37e6c8b9898ad580b4666407cb324fe8cd6e18dd1860fa811028b34575c577f
Parent: 475a1d58a15fb70519407398d01984eb1a14a50842e3abe7646cc78a4d96191f
Subject: Refactor the manifest to make it modular (#478) (08a74eb845ea)
Date: 2026-06-09 00:14:46 +0000
```

Runtime details:
```
flatpak info org.gnome.Platform

GNOME Application Platform version 50 - Shared libraries used by GNOME
applications

ID: org.gnome.Platform
Ref: runtime/org.gnome.Platform/x86_64/50
Arch: x86_64
Branch: 50
License: GPL-2.0+
Origin: flathub
Collection: org.flathub.Stable
Installation: system
Installed: 1.1 GB

Commit: de3b0b88d9e931daa384fe5444690c68496fa770ddc30e9e14df7bdecc9a4fdc
Parent: f780e089a0b3bfbde2b3d75e5f23d084583c4ca67d4f6c72f70603c06d486ac2
Subject: Build of the GNOME Runtime from: 50.3-12-g3c3f235ee
Date: 2026-08-02 16:58:04 +0000

```

My system:

```
lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 24.04.4 LTS
Release: 24.04
Codename: noble

uname -a
Linux REDACTED 6.8.0-136-generic #136-Ubuntu SMP PREEMPT_DYNAMIC Wed Jul 1 21:53:05 UTC 2026 x86_64 x86_64 x86_64 GNU/Linux
```

Start by reproducing the GUI launch with the reported Flatpak build and compare it with launching Octave without --gui. Inspect the Flatpak manifest and GNOME runtime details for the source of the Clutter assertion; the issue is done when the GUI window renders normally without that assertion.

Written by the indexing model from the issue text.