Store new desktop app data outside the fyne subdirectory

Apps that already have data under the "fyne" directory keep using it,
so nothing is moved behind the user's back. Apps without any data there
get ~/.config/app_id and ~/.cache/app_id (or the platform equivalent).
The Fyne-wide settings.json and theme.json stay where they are.

Fixes #6464

Author: 4RH1T3CT0R7

app/cache_other.go

@@ -11,5 +11,5 @@ import (
 
 func rootCacheDir(a fyne.App) string {
 	desktopCache, _ := os.UserCacheDir()
-	return filepath.Join(desktopCache, "fyne", a.UniqueID())
+	return a.(*fyneApp).appDir(filepath.Join(desktopCache, "fyne"))
 }

app/preferences_other.go

@@ -3,6 +3,7 @@
 package app
 
 import (
+	"os"
 	"path/filepath"
 
 	"fyne.io/fyne/v2/internal/app"
@@ -15,7 +16,23 @@ func (p *preferences) storagePath() string {
 
 // storageRoot returns the location of the app storage
 func (a *fyneApp) storageRoot() string {
-	return filepath.Join(app.RootConfigDir(), a.UniqueID())
+	return a.appDir(app.RootConfigDir())
+}
+
+// appDir returns the directory for this app's data given the "fyne" directory
+// that used to hold all apps. Apps that already have data in there keep using
+// it, new apps live directly under the parent (e.g. ~/.config/app_id).
+// Apps without an ID have nothing to migrate, so they stay in the old place,
+// as does everything when the root is not a "fyne" directory (test builds).
+func (a *fyneApp) appDir(fyneDir string) string {
+	old := filepath.Join(fyneDir, a.UniqueID())
+	if a.missingID || filepath.Base(fyneDir) != "fyne" {
+		return old
+	}
+	if _, err := os.Stat(old); err == nil {
+		return old
+	}
+	return filepath.Join(filepath.Dir(fyneDir), a.UniqueID())
 }
 
 func (p *preferences) watch() {

app/preferences_other_test.go

@@ -0,0 +1,38 @@
+//go:build !ios && !android && !mobile && !wasm
+
+package app
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestFyneApp_appDir(t *testing.T) {
+	parent := t.TempDir()
+	fyneDir := filepath.Join(parent, "fyne")
+	testDir := filepath.Join(parent, "fyne-test")
+	require.NoError(t, os.MkdirAll(filepath.Join(fyneDir, "io.fyne.old"), 0o700))
+
+	noID, noIDOld := &fyneApp{}, &fyneApp{}
+	require.NoError(t, os.MkdirAll(filepath.Join(fyneDir, noIDOld.UniqueID()), 0o700))
+
+	for name, tt := range map[string]struct {
+		app  *fyneApp
+		root string
+		want string
+	}{
+		"existing app keeps fyne subdirectory":      {&fyneApp{uniqueID: "io.fyne.old"}, fyneDir, filepath.Join(fyneDir, "io.fyne.old")},
+		"new app moves out of fyne subdirectory":    {&fyneApp{uniqueID: "io.fyne.new"}, fyneDir, filepath.Join(parent, "io.fyne.new")},
+		"root not named fyne is left alone":         {&fyneApp{uniqueID: "io.fyne.new"}, testDir, filepath.Join(testDir, "io.fyne.new")},
+		"app without ID stays in fyne subdirectory": {noID, fyneDir, filepath.Join(fyneDir, noID.UniqueID())},
+		"app without ID keeps existing data":        {noIDOld, fyneDir, filepath.Join(fyneDir, noIDOld.UniqueID())},
+	} {
+		t.Run(name, func(t *testing.T) {
+			assert.Equal(t, tt.want, tt.app.appDir(tt.root))
+		})
+	}
+}