Fix #123586: Sequencer: Box select in Preview doesn't work after selecting it in the menu

select_box operator needs different context in preview and sequencer
regions. It wasn't specified, so it defaulted to sequencer context and
wasn't working in preview.

This PR sets poll for region and sets correct context for preview. This
also means shortcut appears on menu item in preview now.

Pull Request: https://projects.blender.org/blender/blender/pulls/123757

scripts/startup/bl_ui/space_sequencer.py

@@ -573,7 +573,7 @@ class SEQUENCER_MT_select(Menu):
     def draw(self, context):
         layout = self.layout
         st = context.space_data
-        has_sequencer, _has_preview = _space_view_types(st)
+        has_sequencer, has_preview = _space_view_types(st)
         is_retiming = context.scene.sequence_editor.selected_retiming_keys
 
         layout.operator("sequencer.select_all", text="All").action = 'SELECT'
@@ -582,12 +582,14 @@ class SEQUENCER_MT_select(Menu):
 
         layout.separator()
 
-        layout.operator("sequencer.select_box", text="Box Select")
-
         col = layout.column()
         if has_sequencer:
+            col.operator("sequencer.select_box", text="Box Select")
             props = col.operator("sequencer.select_box", text="Box Select (Include Handles)")
             props.include_handles = True
+        elif has_preview:
+            col.operator_context = 'INVOKE_REGION_PREVIEW'
+            col.operator("sequencer.select_box", text="Box Select")
 
         col.separator()