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.
This commit will use the ~/Downloads directory as the default path,
which seems like a better choice then dumping files into the user's home dir.

Author: pelya

app/main.ts

@@ -5,6 +5,8 @@ import { join, normalize, extname, dirname, basename } from 'path';
 import { pathToFileURL } from 'url';
 import * as os from 'os';
 import { chmod, realpath, writeFile } from 'fs-extra';
+import { exec } from 'child_process';
+import { promisify } from 'util';
 import { randomBytes } from 'crypto';
 import { createParser } from 'dashdash';
 
@@ -3021,6 +3023,32 @@ ipc.handle('show-save-dialog', async (_event, { defaultPath }) => {
     return { canceled: true };
   }
 
+  if (
+    process.platform === 'linux' &&
+    defaultPath &&
+    defaultPath.charAt(0) !== '/'
+  ) {
+    // 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 = (
+        await promisify(exec)('xdg-user-dir DOWNLOAD')
+      ).stdout.trim();
+      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 = process.env['HOME'] || '';
+      getLogger().info(
+        'show-save-dialog: saving to user home directory: ' + downloadsPath
+      );
+    }
+    if (downloadsPath) {
+      defaultPath = downloadsPath + '/' + defaultPath;
+    }
+  }
+
   const { canceled, filePath: selectedFilePath } = await dialog.showSaveDialog(
     mainWindow,
     {