Stephan-P Thank you for your feedback!
- Kp2A does not offer autofill on username field.
...seems to be website specific and is something that might be addressed by the package maintainer.
I did a few more tests and found that for example, on https://discuss.grapheneos.org/ Keepass2Android does not fill the username but on https://community.brave.com/ it works flawlessly with both the username and password fields. It is annoying but at least in can be overcome by the password managers' specific keyboards (Keepass2Android keyboard or Magikeyboard in case of KeePassDX).
I would prefer to use KeePassDX but my conclusion thus far is that its autofill doesn't work in the majority of cases (besides on Gecko-based web browsers). Keepass2Android instead works even without Google Play services and has just some minor issues.
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Vivaldi is a very nicely built web browser but sadly cannot be configured in such way to work independently from Google services. This close attachment to Google rules it out for me personally; see following analysis of the desktop version (won't be a lot different on mobile OS). Translate with your preferred service but Google might be blocked: https://www.kuketz-blog.de/vivaldi-datensendeverhalten-desktop-version-browser-check-teil5/
Furthermore it remains to be seen how Vivaldi manages the Google Manifest V3 "update" ( https://vivaldi.com/blog/manifest-v3-update-vivaldi-is-future-proofed-with-its-built-in-functionality/ ) vs. how Brave ( https://brave.com/blog/brave-shields-manifest-v3/ ) and others do.
Vanadium is really great especially for its hardening and user friendliness but lacks state-of-the-art fingerprinting protections and a comprehensive Ads-/Trackers/Malware blocker.
Brave delivers in both ways, with possibly the strongest fingerprinting protections just after Tor- and Mullvad Browsers (Mullvad only for desktop OS) and can be configured for privacy friendly use (additional blocking of "variations.brave.com" and "star-randsrv.bsg.brave.com" through a DNS filter required).
In any case using Aurora Store, the following might require mitigation, depending on ones personal needs: https://gitlab.com/AuroraOSS/AuroraStore/-/blob/master/POLICY.md
Gecko-based web browsers like Firefox and Fennec instead, don't include Site Isolation on Android ( https://www.chromium.org/Home/chromium-security/site-isolation/ ) and don't set the isolatedProcess flag ( https://developer.android.com/guide/topics/manifest/service-element#isolated ). They only offer Site Isolation on desktop OS ( https://wiki.mozilla.org/Project_Fission ). Hence, of all the available options only the Tor Browser remains a good choice for its specific use cases.
I'd wish KeePassDX would at least work to Keepass2Android's extent. :-/