White flash when opening browser from external link (xdg-open) — worse with light theme + dark page

[sporteka2]

When clicking a link in a terminal emulator (or any external application) while Firefox/Floorp is set as the default browser, the browser opens and briefly shows a solid white screen before loading the actual page. This creates a distracting white flash.

The flash is much worse when the system uses a light theme and the opened page is dark-themed (e.g. dark mode websites). The screen goes solid white first, then transitions to the dark page — a jarring flashbang effect.

This happens because the browser window appears before the page content is rendered, and the window background is white.

Steps to reproduce

1. Use Cinnamon with a light theme
2. Set Firefox as default browser
3. Open a terminal and run: xdg-open https://example.com (or any URL)
4. Observe: a white window appears briefly before the page content loads

Expected behavior
The browser window should not flash white before showing the page content, especially when a dark page is about to be loaded.

Environment

• Cinnamon version: (latest from Linux Mint)
• Firefox version: 152.0
• X11 session
• System theme: light

Related Firefox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=2054825
Related Floorp issue: Floorp-Projects/Floorp#2533

[sporteka2]

Additional observation: when links from external apps open in an existing tab (instead of a new window), the problem is even worse — the previous tab's content is briefly visible first, then the tab switches to the new URL and shows the white flash. So the user sees: previous page → white flash → actual content.