Bug 1439834 - Draw titlebar with some extent, r=dao

Some themes (Adwaita for instance) draws bold dark line at
titlebar bottom. It does not fit well with Firefox tabbar UI so
draw themed titlebar with some extent to make the titlebar
bottom part invisible (it's clipped by cairo).

MozReview-Commit-ID: 3rs4UzFJdPa

--HG--
extra : rebase_source : ca9270f549a3106711afac8ee0c7a30839ab2bf3

widget/gtk/gtk3drawing.cpp

@@ -2285,9 +2285,15 @@ moz_gtk_header_bar_paint(WidgetNodeType widgetType,
     GtkStyleContext *style = GetStyleContext(widgetType, GTK_TEXT_DIR_LTR,
                                              state_flags);
     InsetByMargin(rect, style);
-    gtk_render_background(style, cr, rect->x, rect->y, rect->width,
-                          rect->height);
-    gtk_render_frame(style, cr, rect->x, rect->y, rect->width, rect->height);
+
+    // Some themes (Adwaita for instance) draws bold dark line at
+    // titlebar bottom. It does not fit well with Firefox tabs so
+    // draw with some extent to make the titlebar bottom part invisible.
+    #define TITLEBAR_EXTENT 4
+    gtk_render_background(style, cr, rect->x, rect->y,
+                          rect->width, rect->height + TITLEBAR_EXTENT);
+    gtk_render_frame(style, cr, rect->x, rect->y,
+                     rect->width, rect->height + TITLEBAR_EXTENT);
 
     return MOZ_GTK_SUCCESS;
 }