https://bugs.kde.org/show_bug.cgi?id=520521

--- Comment #2 from xynrin <[email protected]> ---
### Description
After upgrading to Fedora 44 (KDE Plasma 6.6.5, Wayland session), icons for
several third-party applications (e.g., CLion, PyCharm, Omniget, Cockpit Tools)
in the Task Manager/System Tray appear as generic gray document icons with
question marks (or completely blank on some themes). 

The root cause was tracked down to the icon lookup engine failing to resolve
fallback icons. Third-party apps place their icons in
`/usr/share/icons/hicolor/`. In `hicolor/index.theme`, `Hidden=true` is set by
default. It appears that the Qt6 icon loader or KDE's `KIconLoader` completely
skips scanning the `hicolor` directory when initializing or processing the
theme inheritance chain if `Hidden=true` is present, causing a broken
inheritance chain and returning `null` to the QML rendering delegates.

### Environment
- OS: Fedora Linux 44
- DE: KDE Plasma 6.6.5 (Wayland session)
- WM: KWin
- Graphics: NVIDIA GeForce RTX 5060 Mobile (Using proprietary drivers)
- Icon Themes tested: Papirus-Dark, Breeze, Breeze-Light

### Steps to Reproduce
1. Install a third-party application whose icon only exists in
`/usr/share/icons/hicolor/...` (and not pre-baked into themes like Breeze or
Papirus).
2. Pin the application to the Task Manager or launch it under a Wayland
session.
3. Observe that the icon displays as a gray question mark.
4. Check `journalctl --user`, notice repeating QML errors:
`ToolTipDelegate.qml: TypeError: Cannot read property 'containsMouse' of null`
when hovering over the broken icon.
5. Running `kiconfinder6 <icon-name>` returns not found, despite the
`.png/.svg` file existing inside the `hicolor` subdirectories.

### Underlying Root Cause
Using `kiconfinder6` to trace the lookup chain reveals that themes failing to
explicitly declare inheritance properly, or even standard themes trying to fall
back to the system default, get choked off at the `hicolor` level. 

According to XDG Icon Theme Specifications, `Hidden=true` means the theme
should not be selected in the GUI configuration, but it MUST still be used as a
fallback lookup. Currently, Plasma 6 treats `Hidden=true` as "Do not load this
theme structure into memory hash/cache at all," causing a total regression of
the XDG fallback mechanism.

### Workaround
The issue can be verified and temporarily resolved by physically copying the
missing icons from the `hicolor` tree directly into the active icon theme
directory (e.g., `~/.local/share/icons/Papirus-Dark/`), forcing the lookup
loader to hit the icons at stage 1 before falling back to `hicolor`.

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