Use an absolute filename for the file save dialog on Linux

This fixes #6912.
On Gnome the save dialog works with any filename, on KDE Plasma
the save dialog opens with an empty filename input field,
unless the application uses an absolute file path instead of only the filename.
The default save location is the Downloads directory in the user's home directory.

app/main.ts

@@ -1,7 +1,7 @@
 // Copyright 2017 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
-import { join, normalize, extname, dirname, basename } from 'path';
+import { join, normalize, extname, dirname, basename, isAbsolute } from 'path';
 import { pathToFileURL } from 'url';
 import * as os from 'os';
 import { chmod, realpath, writeFile } from 'fs-extra';
@@ -3021,6 +3021,30 @@ ipc.handle('show-save-dialog', async (_event, { defaultPath }) => {
     return { canceled: true };
   }
 
+  if (
+    process.platform === 'linux' &&
+    defaultPath &&
+    !isAbsolute(defaultPath)
+  ) {
+    // On Linux the defaultPath should be an absolute path, otherwise the save dialog will have an empty filename on KDE/Plasma
+    let downloadsPath = '';
+    try {
+      downloadsPath = app.getPath('downloads');
+      getLogger().info(
+        'show-save-dialog: saving to user downloads directory: ' + downloadsPath
+      );
+    } catch (e) {
+      // If we cannot get Downloads path, fall back to the user's home directory
+      downloadsPath = app.getPath('home');
+      getLogger().info(
+        'show-save-dialog: saving to user home directory: ' + downloadsPath
+      );
+    }
+    if (downloadsPath) {
+      defaultPath = join(downloadsPath, defaultPath)
+    }
+  }
+
   const { canceled, filePath: selectedFilePath } = await dialog.showSaveDialog(
     mainWindow,
     {