commit 5e689ba81d8f35fb4bd2328eb2ee806290ae5e5b
Author: Campbell Barton <ideasman42@gmail.com>
Date:   Wed Feb 6 13:36:23 2019 +1100

    Preferences: switch header alignment when setting the option
    
    This was only used for newly created windows,
    however it's more useful when the existing window is edited.

diff --git a/source/blender/makesrna/intern/rna_userdef.c b/source/blender/makesrna/intern/rna_userdef.c
index 2b06ae06dea..551b625843d 100644
--- a/source/blender/makesrna/intern/rna_userdef.c
+++ b/source/blender/makesrna/intern/rna_userdef.c
@@ -160,6 +160,24 @@ static void rna_userdef_language_update(Main *UNUSED(bmain), Scene *UNUSED(scene
 	UI_reinit_font();
 }
 
+static void rna_userdef_update_header_default(Main *bmain, Scene *scene, PointerRNA *ptr)
+{
+	int alignment = (U.uiflag & USER_HEADER_BOTTOM) ? RGN_ALIGN_BOTTOM : RGN_ALIGN_TOP;
+	for (bScreen *screen = bmain->screen.first; screen; screen = screen->id.next) {
+		for (ScrArea *sa = screen->areabase.first; sa; sa = sa->next) {
+			if (ELEM(sa->spacetype, SPACE_FILE, SPACE_USERPREF, SPACE_OUTLINER, SPACE_BUTS)) {
+				continue;
+			}
+			for (ARegion *ar = sa->regionbase.first; ar; ar = ar->next) {
+				if (ar->regiontype == RGN_TYPE_HEADER) {
+					ar->alignment = alignment;
+				}
+			}
+		}
+	}
+	rna_userdef_update_ui(bmain, scene, ptr);
+}
+
 static void rna_userdef_script_autoexec_update(Main *UNUSED(bmain), Scene *UNUSED(scene), PointerRNA *ptr)
 {
 	UserDef *userdef = (UserDef *)ptr->data;
@@ -3867,7 +3885,7 @@ static void rna_def_userdef_view(BlenderRNA *brna)
 	RNA_def_property_enum_items(prop, header_align_default_items);
 	RNA_def_property_enum_bitflag_sdna(prop, NULL, "uiflag");
 	RNA_def_property_ui_text(prop, "Header Position", "Default header position for new space-types");
-	RNA_def_property_update(prop, 0, "rna_userdef_update");
+	RNA_def_property_update(prop, 0, "rna_userdef_update_header_default");
 
 	static const EnumPropertyItem text_hinting_items[] = {
 		{0, "AUTO", 0, "Auto", ""},