BaseApp 6.9 breaks NVIDIA GPU support (6.8 works, native Qt 6.9 works fine)

## Problem
`io.qt.qtwebengine.BaseApp` version 6.9 shows black screen or white screen but no content with NVIDIA GPUs, but version 6.8 works perfectly. Native Qt 6.9 applications also work fine with the same NVIDIA drivers.

## Environment
- **GPU:** NVIDIA GTX 845M (Maxwell architecture)
- **Drivers tested:** 535, 570, 590 (all fail in BaseApp 6.9)
- **OS:** Gentoo Linux
- **Flatpak version:** Flatpak 1.16.0

## Test Results

| Environment | Qt Version | NVIDIA Result |
|-------------|------------|---------------|
| Qt Creator (native build) | 6.8 | ✅ Works |
| Qt Creator (native build) | 6.9 | ✅ Works |
| Flatpak BaseApp | 6.8 | ✅ Works |
| Flatpak BaseApp | 6.9 | ❌ Black screen |

**Intel Mesa works fine in all cases.**

## Errors in BaseApp 6.9
Only --use-gl=angle is supported on this platform.
GBM is not supported with the current configuration. Fallback to Vulkan rendering in Chromium.
[2:15:1026/233601.406645:ERROR:bus.cc(407)] Failed to connect to the bus
[2:16:1026/233605.482931:ERROR:validation_errors.cc(117)] Invalid message: VALIDATION_ERROR_MESSAGE_HEADER_UNKNOWN_METHOD
[2:16:1026/233605.482952:ERROR:render_process_host_impl.cc(5540)] Terminating render process for bad Mojo message

## Attempted Workarounds (all failed)

```bash
--env=QTWEBENGINE_CHROMIUM_FLAGS="--disable-gpu"
--env=QTWEBENGINE_CHROMIUM_FLAGS="--use-gl=angle --use-angle=default"
--env=QTWEBENGINE_CHROMIUM_FLAGS="--use-gl=angle --use-angle=vulkan"
--env=QTWEBENGINE_CHROMIUM_FLAGS="--use-gl=angle --use-angle=gl"
--env=QTWEBENGINE_CHROMIUM_FLAGS="--use-gl=angle --use-angle=swiftshader"
--env=QTWEBENGINE_CHROMIUM_FLAGS="--use-gl=egl-angle"
None of these flags resolve the issue.
Analysis
Since native Qt 6.9 works perfectly with NVIDIA, this appears to be a BaseApp 6.9 packaging issue, not a Qt 6.9 or NVIDIA driver problem.
Possible causes:
Missing NVIDIA-specific Flatpak extensions in 6.9
Changed device permissions (/dev/dri/*)
Missing or misconfigured GL libraries
ANGLE configuration incompatible with NVIDIA in sandbox
Current Workaround
Using base-version: '6.8' in app manifest works perfectly.
Request
Could you please:
1.
Compare BaseApp 6.8 and 6.9 manifests for NVIDIA-related changes
2.
Check if NVIDIA GL extensions are properly included in 6.9
3.
Verify device permissions and sandbox configuration
Thank you!