Robustify Linux PDF export against file dialog quirks

[denisfalqueto]

Summary

Fixes two related symptoms reported on Linux (Arch/Manjaro/Fedora/CachyOS,
mostly under KDE Plasma Wayland with Qt 6.11):

• The file save/export dialog doesn't respect the desktop's xdg-desktop-portal
  configuration.
• PDF export silently fails (no file created) or, in some environments,
  crashes — including when the file name is typed with the extension.

Resolves #33341, resolves #33559.

Root cause 1 — theme forced unconditionally

src/app/main.cpp unconditionally forces QT_QPA_PLATFORMTHEME=gtk3 on
Linux. This overrides any theme the user has explicitly configured (e.g.
xdgdesktopportal, for proper native/sandboxed dialog support via the
portal), which is a likely contributor to dialogs not respecting the
desktop environment and to instability in some Wayland setups.

Fix: only apply the gtk3 default when the user hasn't already set
QT_QPA_PLATFORMTHEME. The default is unchanged for everyone who doesn't
set it.

I want to flag why gtk3 is forced in the first place, for reviewers:
it was introduced in #13847 to fix #13113 ("keyboard doesn't work in file
dialogs"), where the legacy Qt widget dialog swallowed keyboard input.
This PR does not change that default, only whether it can be
overridden — so #13113 should not be reintroduced. I also intentionally
left QT_QPA_PLATFORM=xcb untouched; it was introduced in #28461 to fix
unrelated Wayland issues (VST crash #28446, panel docking #28352) and
removing it risks regressing those.

Root cause 2 — missing extension breaks writer lookup

On some Linux native/portal file dialogs, the extension isn't
auto-appended when the user doesn't type one. exportScores() in
src/project/internal/exportprojectscenario.cpp picks the writer by the
returned path's suffix (io::suffix()), so a missing suffix meant no
writer was found and the export silently failed.

Fix: after the dialog returns, force the expected suffix (from
exportType.suffixes) if the returned path doesn't already have one of
the known suffixes for that export format.

A companion PR in muse_framework
(musescore/muse_framework#139) makes the dialog itself request a
defaultSuffix for the single-format case, addressing the same root
cause closer to the source. This PR's fix is independent of the dialog
backend and acts as a safety net either way.

Note: I looked into also fixing the analogous "Save As" ambiguity for
.mscz vs. the experimental uncompressed-folder option
(src/project/internal/opensaveprojectscenario.cpp), but
selectSavingFileSync() doesn't currently report which of the two
filters the user selected, making a suffix-based heuristic there
genuinely ambiguous (forcing .mscz on empty suffix would break the
folder option for users who select it correctly). Left that out of scope
for this PR rather than risk a different regression.

Testing

Built locally (Debug) and manually tested on KDE Plasma Wayland, with the
companion muse_framework branch checked out:

1. Export to PDF without typing an extension → now correctly produces
   name.pdf (previously failed/crashed).
2. Export to PDF typing .pdf explicitly → still works, no duplicated
   suffix.
3. Keyboard interaction inside the file dialog (arrow keys, typing text,
   Enter to confirm) → works normally, no regression of [MU4 Issue] keyboard doesn't work in file dialogs  #13113.
4. App starts and runs without crashing.

Disclaimer

This change was implemented with the assistance of Claude Code (Anthropic),
which performed the code changes, the build, and the testing steps
described above. A human reviewed and validated the entire process step by
step, and endorses the final result.

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Reviewing files that changed from the base of the PR and between 0bbf8ab and dfcc9bf.

📒 Files selected for processing (2)

• src/app/main.cpp
• src/project/internal/exportprojectscenario.cpp

🚧 Files skipped from review as they are similar to previous changes (2)

• src/app/main.cpp
• src/project/internal/exportprojectscenario.cpp

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📝 Walkthrough

Walkthrough

This PR contains two independent fixes. In main.cpp, the logic that sets QT_QPA_PLATFORMTHEME to gtk3 now also checks whether that environment variable is already set, in addition to checking that the platform is not offscreen. In exportprojectscenario.cpp, askExportPath() now validates the selected export path suffix against known export suffixes and appends the first valid suffix when the current suffix is unrecognized.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately reflects the Linux file-dialog/export fixes, even if it omits the suffix fallback detail.
Description check	✅ Passed	The description is detailed and mostly matches the template's intent, but it omits the checklist section.
Linked Issues check	✅ Passed	The theme override and suffix fix address the Linux/AppImage export failures and preserve file-dialog keyboard behavior for [#33341, #33559, #13113].
Out of Scope Changes check	✅ Passed	The changes stay within the export/file-dialog scope and do not introduce unrelated functionality.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[denisfalqueto]

I just did a rebase on branch master to get some fixes for my personal use.