apprt/gtk: make window-decoration=false with gtk-titlebar=true look b…

…etter (#3999)

Before this change, there seemed to be some artifacting in the window
corners due to the window border no longer outlining the content
properly. By detecting the situation, we can turn the window border
radius off.

src/apprt/gtk/App.zig

@@ -972,6 +972,9 @@ fn loadRuntimeCss(
     const headerbar_foreground = config.@"window-titlebar-foreground" orelse config.foreground;
 
     try writer.print(
+        \\window.without-window-decoration-and-with-titlebar {{
+        \\  border-radius: 0 0;
+        \\}}
         \\widget.unfocused-split {{
         \\ opacity: {d:.2};
         \\ background-color: rgb({d},{d},{d});

src/apprt/gtk/Window.zig

@@ -209,6 +209,11 @@ pub fn init(self: *Window, app: *App) !void {
     // If we are disabling decorations then disable them right away.
     if (!app.config.@"window-decoration") {
         c.gtk_window_set_decorated(gtk_window, 0);
+
+        // Fix any artifacting that may occur in window corners.
+        if (app.config.@"gtk-titlebar") {
+            c.gtk_widget_add_css_class(window, "without-window-decoration-and-with-titlebar");
+        }
     }
 
     // In debug we show a warning and apply the 'devel' class to the window.