fixes #1077

main/ui/src/main/java/org/cryptomator/ui/mainwindow/MainWindowController.java

@@ -6,6 +6,7 @@ import javafx.fxml.FXML;
 import javafx.scene.input.DragEvent;
 import javafx.scene.input.TransferMode;
 import javafx.scene.layout.StackPane;
+import org.apache.commons.lang3.SystemUtils;
 import org.cryptomator.common.vaults.VaultListManager;
 import org.cryptomator.ui.common.FxController;
 import org.cryptomator.ui.wrongfilealert.WrongFileAlertComponent;
@@ -46,6 +47,9 @@ public class MainWindowController implements FxController {
 		root.setOnDragOver(this::handleDragEvent);
 		root.setOnDragDropped(this::handleDragEvent);
 		root.setOnDragExited(this::handleDragEvent);
+		if (SystemUtils.IS_OS_WINDOWS) {
+			root.getStyleClass().add("os-windows");
+		}
 	}
 
 	private void handleDragEvent(DragEvent event) {

main/ui/src/main/resources/css/dark_theme.css

@@ -148,6 +148,12 @@
  *                                                                             *
  ******************************************************************************/
 
+/* windows needs an explicit border: */
+.main-window.os-windows {
+	-fx-border-color: TITLE_BG;
+	-fx-border-width: 1px;
+}
+
 .main-window .title {
 	-fx-background-color: CONTROL_BORDER_NORMAL, TITLE_BG;
 	-fx-background-insets: 0, 0 0 1px 0;

main/ui/src/main/resources/css/light_theme.css

@@ -148,6 +148,12 @@
  *                                                                             *
  ******************************************************************************/
 
+/* windows needs an explicit border: */
+.main-window.os-windows {
+	-fx-border-color: TITLE_BG;
+	-fx-border-width: 1px;
+}
+
 .main-window .title {
 	-fx-background-color: TITLE_BG;
 }