fix(qt): add non-native file dialog option

[thepastaclaw]

PR Body

Summary

• Add a -nativefiledialogs GUI option.
• Default remains the existing native-dialog behavior.
• With -nativefiledialogs=0, central save/open dialogs use Qt dialogs.
• Route the data-directory chooser through the same GUIUtil wrapper.

Fixes #6935.

Motivation

Some Linux desktop environments can hang or freeze in native Qt file dialogs
before Dash reaches the export or wallet-backup code paths.

This keeps the default UX unchanged while adding a narrow escape hatch for
affected users.

Validation

• git diff --check upstream/develop...HEAD
• code-review dashpay/dash upstream/develop fix/file-dialog-nonnative-option
  returned ship
• Reviewed all changed files.
• Confirmed current static QFileDialog save/open/directory call sites under
  src/qt route through GUIUtil on this branch.

No GUI runtime test was run in this macOS worktree. The reported freeze requires
an affected Linux/KDE/native-dialog environment to reproduce.

[thepastaclaw]

@coderabbitai review

[coderabbitai]

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Review triggered.

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[github-actions]

✅ No Merge Conflicts Detected

This PR currently has no conflicts with other open PRs.

[thepastaclaw]

🕓 Ready for review — 2 ahead in queue (commit e1c4012)
Queue position: 3/84

[coderabbitai]

Walkthrough

This PR introduces a new -nativefiledialogs CLI option (defaulting to true) that controls whether Qt file dialogs use the native system dialogs or the non-native fallback. A centralized GetFileDialogOptions() helper function reads this option and returns the appropriate Qt file dialog flags. This helper is applied to three file dialog functions: the existing getSaveFileName() and getOpenFileName() wrappers, and a new getExistingDirectory() wrapper. The data directory selector in the intro screen is updated to use the new wrapper instead of calling Qt directly.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 20.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding a non-native file dialog option to the Qt GUI.
Description check	✅ Passed	The description is well-structured, explaining the motivation, changes, and validation. It directly relates to the changeset and issue being addressed.
Linked Issues check	✅ Passed	The code changes fully address issue #6935 by providing an escape hatch to disable native file dialogs that freeze on affected Linux/KDE systems, matching the reported problem.
Out of Scope Changes check	✅ Passed	All changes are directly related to implementing the non-native file dialog option and routing relevant dialog calls through the GUIUtil wrapper, with no extraneous modifications.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/qt/intro.cpp`:
- Line 328: The call to GUIUtil::getExistingDirectory in
on_ellipsisButton_clicked uses a bare string and nullptr parent; wrap the
caption in tr() and pass the Intro widget as the parent instead of nullptr.
Update the invocation of GUIUtil::getExistingDirectory(nullptr, "Choose data
directory", ui->dataDirectory->text()) to use tr("Choose data directory") for
translation and use this (the Intro/QWidget instance) as the parent so the file
chooser is localized and properly parented to the Intro dialog.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 0521312b-907f-4b6d-8ac1-9d711b04bc65

📥 Commits

Reviewing files that changed from the base of the PR and between 5fd84aa and 0639e13.

📒 Files selected for processing (4)

• src/qt/bitcoin.cpp
• src/qt/guiutil.cpp
• src/qt/guiutil.h
• src/qt/intro.cpp

[thepastaclaw]

The remaining red linux64-test failure is feature_asset_locks.py timing out while waiting for an llmq_test_platform quorum. This is unrelated to this PR’s Qt-only diff and is now tracked separately in #7310.

[thepastaclaw]

Code Review

Small Qt GUI change adding -nativefiledialogs (default true) to opt out of native file dialogs on desktop environments where they hang. The wrapper plumbing is consistent and the default preserves existing behavior. Codex found no issues; only one minor maintainability nit retained.

Reviewed commit: e1c4012

💬 1 nitpick(s)

[thepastaclaw]

💬 Nitpick: Default value for -nativefiledialogs is duplicated

The default true is hardcoded in two places: in SetupUIArgs at src/qt/bitcoin.cpp:509 (the help text uses strprintf(... %u, true)) and in GetFileDialogOptions() at src/qt/guiutil.cpp:502 (gArgs.GetBoolArg("-nativefiledialogs", /*fDefault=*/true)). If the default ever changes, both sites must be updated in lock-step. Hoisting a static constexpr bool DEFAULT_NATIVE_FILE_DIALOGS = true; (alongside DEFAULT_SPLASHSCREEN / similar) and reusing it in both locations would single-source the value. Not a correctness issue — the surrounding code (e.g. -splash two lines below) follows the same %u + bool pattern, so this is consistent with existing style.

_{source: ['claude']}