snap: Fixed issues with storing the default session

Also added a workaround to help people who've already installed and ran
the snap.

Closes mapeditor#2852

src/tiled/preferences.cpp

@@ -607,7 +607,14 @@ QString Preferences::startupSession() const
 
 void Preferences::setLastSession(const QString &fileName)
 {
-    setValue(QLatin1String("Project/LastSession"), fileName);
+    // Don't store the path to the default session, since this path may vary
+    // between restarts. For example the snap release includes the build
+    // number in the path and trying to save to a session file for a different
+    // version doesn't work.
+    if (fileName == Session::defaultFileName())
+        setValue(QLatin1String("Project/LastSession"), QString());
+    else
+        setValue(QLatin1String("Project/LastSession"), fileName);
 }
 
 bool Preferences::restoreSessionOnStartup() const

src/tiled/session.cpp

@@ -213,7 +213,21 @@ QString Session::defaultFileNameForProject(const QString &projectFile)
 Session &Session::initialize()
 {
     Q_ASSERT(!mCurrent);
-    return switchCurrent(Preferences::instance()->startupSession());
+    auto &session = switchCurrent(Preferences::instance()->startupSession());
+
+    // Workaround for users facing issue #2852, bringing their default session
+    // to the right location.
+    if (session.project.isEmpty()) {
+        if (QFileInfo(session.fileName()).fileName() == QLatin1String("default.tiled-session")) {
+            const QString defaultName = defaultFileName();
+            if (session.fileName() != defaultName) {
+                session.setFileName(defaultName);
+                Preferences::instance()->setLastSession(defaultName);
+            }
+        }
+    }
+
+    return session;
 }
 
 Session &Session::current()