The border of the widget inside `Panel::show_inside` is not displayed correctly.

[zhatuokun]

Describe the bug

When the inner_margin is not set in the Panel, the border of the widget inside Panel::show_inside is not displayed correctly.

To Reproduce

Run this code

      CentralPanel::default().show(ctx, |ui| {
          TopBottomPanel::top("top")
              .frame(Frame::none())
              .show_inside(ui, |ui| {
                  ui.button("text");
                  ui.button("text");
              });
  
          CentralPanel::default()
              .frame(Frame::none())
              .show_inside(ui, |ui| {
                  ui.button("text");
                  ui.button("text");
  
                  let mut s = "".to_string();
                  ui.text_edit_singleline(&mut s);
              });
      });

Expected behavior

Border is displayed correctly.

Screenshots

https://github.com/user-attachments/assets/a34d9cac-8ed2-40ad-9519-724cb8820c60

Desktop (please complete the following information):

• OS: Windows 11
• Version: 23H2

Additional context