Avoid gap between titlebar and body on linux (#40228)

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: John Tur <john-tur@outlook.com>

Release Notes:

- N/A

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: John Tur <john-tur@outlook.com>

crates/title_bar/src/platform_title_bar.rs

@@ -97,6 +97,7 @@ impl Render for PlatformTitleBar {
                     })
                     // this border is to avoid a transparent gap in the rounded corners
                     .mt(px(-1.))
+                    .mb(px(-1.))
                     .border(px(1.))
                     .border_color(titlebar_color),
             })