Microsoft account picker triggers "invalid url: relative URL without a base" on Linux (Wayland, KDE)

[DarkDon]

Please confirm the following.

• I checked the existing issues for duplicate problems
• I have tried resolving the issue using the support portal
• I have ensured my Modrinth App installation is up to date

What version of the Modrinth App are you using?

0.15.11

What operating systems are you seeing the problem on?

Linux

Describe the bug

OS: CachyOS (Arch-based), KDE Plasma, Wayland

When signing in with a Microsoft account, the Microsoft account picker loads correctly and shows my cached account. Clicking the account (or manually re-entering credentials via "Sign in with another account") triggers the redirect back to the app. At that point, the webview area turns blank/gray, and the app shows the error:

invalid url: relative URL without a base

This happens consistently, both when selecting the pre-filled cached account and when manually typing in credentials — so it's not limited to the SSO/silent-login shortcut.

Steps to reproduce

1. Open Modrinth App, click "Sign in with Microsoft"
2. Microsoft login window opens normally, shows account picker
3. Click on the account (or sign in manually with credentials)
4. Webview area goes blank/gray
5. Error dialog appears: "invalid url: relative URL without a base"

Troubleshooting already attempted (no effect on any of these):

• Full reset of app data (~/.local/share/ModrinthApp, ~/.config/ModrinthApp, ~/.cache/ModrinthApp)
• Clearing WebKitCache, cookies, localstorage, storage folders specifically
• Signing in with a fresh/different account path ("Sign in with another account" instead of cached account)
• Forcing X11 backend: GDK_BACKEND=x11 modrinth-app
• WEBKIT_DISABLE_COMPOSITING_MODE=1 modrinth-app
• WEBKIT_DISABLE_DMABUF_RENDERER=1 modrinth-app
• Verified system clock is NTP-synced (not a time-skew issue)
• Verified no hosts-file or DNS-level blocking of Microsoft/Xbox/Minecraft domains

Happy to provide further debug output (e.g. running the app directly from terminal, screen recording) if useful.

Steps to reproduce

1. Open Modrinth App, click "Sign in with Microsoft"
2. Microsoft login window opens normally, shows account picker
3. Click on the account (or sign in manually with credentials)
4. Webview area goes blank/gray
5. Error dialog appears: "invalid url: relative URL without a base"

Expected behavior

Login into Microsoft Account
Auth via password or password manager

Additional context

Launcher log (launcher_logs/session_*.log) shows no new entries at all when the error occurs — the error appears to originate purely in the webview/frontend layer before any Tauri backend command fires, which is why it's not captured in the log file location I could find.