Fix Import/Export dropdowns not dismissing

They only closed via their own trigger button or a menu item -
clicking outside, pressing Escape, or opening the other dropdown
while one was open left it stuck on screen (confirmed opening both
stacked on top of each other). Add a shared invisible backdrop
(mirroring main.gd's drawer-backdrop pattern) that closes whichever
dropdown is open on outside click or Escape, and have opening one
close the other first.

scripts/ui/flashcards_hub.gd

@@ -35,6 +35,7 @@ var editor_error_label: Label
 
 var export_menu: PanelContainer
 var import_menu: PanelContainer
+var dropdown_backdrop: Control
 
 var import_overlay: Control
 var import_panel: PanelContainer
@@ -53,6 +54,7 @@ func _ready() -> void:
 	size_flags_horizontal = Control.SIZE_EXPAND_FILL
 	size_flags_vertical = Control.SIZE_EXPAND_FILL
 	add_theme_constant_override("separation", 14)
+	_build_dropdown_backdrop()
 	_build_toolbar()
 	_build_filters()
 	status_label = Label.new()
@@ -149,6 +151,18 @@ func _build_toolbar() -> void:
 	])
 
 
+func _build_dropdown_backdrop() -> void:
+	dropdown_backdrop = Control.new()
+	dropdown_backdrop.visible = false
+	dropdown_backdrop.top_level = true
+	dropdown_backdrop.mouse_filter = Control.MOUSE_FILTER_STOP
+	dropdown_backdrop.set_anchors_and_offsets_preset(Control.PRESET_FULL_RECT)
+	dropdown_backdrop.gui_input.connect(func(event: InputEvent):
+		if event is InputEventMouseButton and event.pressed:
+			_close_dropdowns())
+	add_child(dropdown_backdrop)
+
+
 func _build_dropdown(items: Array) -> PanelContainer:
 	var menu := PanelContainer.new()
 	menu.visible = false
@@ -165,20 +179,37 @@ func _build_dropdown(items: Array) -> PanelContainer:
 		button.alignment = HORIZONTAL_ALIGNMENT_LEFT
 		var action: Callable = item[1]
 		button.pressed.connect(func():
-			menu.visible = false
+			_close_dropdowns()
 			action.call())
 		list.add_child(button)
 	return menu
 
 
 func _toggle_dropdown(menu: PanelContainer, anchor: Control) -> void:
-	if menu.visible:
+	var was_visible := menu.visible
-		menu.visible = false
+	_close_dropdowns()
+	if was_visible:
 		return
 	menu.visible = true
+	dropdown_backdrop.visible = true
 	menu.position = anchor.get_global_position() + Vector2(0, anchor.size.y + 4)
 
 
+func _close_dropdowns() -> void:
+	if export_menu != null:
+		export_menu.visible = false
+	if import_menu != null:
+		import_menu.visible = false
+	if dropdown_backdrop != null:
+		dropdown_backdrop.visible = false
+
+
+func _unhandled_input(event: InputEvent) -> void:
+	if event.is_action_pressed("ui_cancel") and dropdown_backdrop != null and dropdown_backdrop.visible:
+		_close_dropdowns()
+		get_viewport().set_input_as_handled()
+
+
 func _build_filters() -> void:
 	var row := HFlowContainer.new()
 	row.add_theme_constant_override("separation", 10)