Created attachment 9136 
Left: after startup, right: after changing theme forth and back.

This happens on xfwm4 4.14.0, and also on current
master (0e91faba6701b4c578fd50f81565d02aaa8ebd09).

When I login and session starts, windows has their title text
slightly moved downwards from center.  Screenshot attached.

And then if I change theme in xfwm4-settings, title text becomes
vertically centered again. Even if I choose the same theme I
started with. It also helps to change DPI in
xfce4-appearance-settings, just increasing and decreasing it
again is enough too.

It's sufficient to set font DPI to 140 in xfce4-appearance-settings
to reproduce the issue. It happens on a clean profile in a clean
installation too. Just non-standard DPI setting causes the title
text to be vertically off-center.

Also found that the following patch fixes the issue on my
installation:

diff --git a/src/frame.c b/src/frame.c
index 34d1466bb..709f7c46d 100644
--- a/src/frame.c
+++ b/src/frame.c
@@ -205,14 +205,8 @@ frameCreateTitlePixmap (Client * c, int state, int left, int right, xfwmPixmap *
     }
     pango_layout_get_pixel_extents (layout, NULL, &logical_rect);
 
-    title_height = screen_info->font_height;
-    if (!title_height)
-    {
-        /* If for some reason the font height is not known,
-         * use the actual pango layout height.
-         */
-        title_height = logical_rect.height;
-    }
+    title_height = logical_rect.height;
+
     title_y = voffset + (frameDecorationTop(screen_info) - title_height) / 2;
     if (title_y + title_height > frameDecorationTop(screen_info))
     {


I suspect that screen_info->font_height is somehow keeps the old
value before DPI is updated at startup. Perhaps, some race
condition. Or maybe signal ordering was changed in GTK 3.
Bisecting points to the patch series with GTK 3 migration. The
issue is not present before those patches, and present immediately
after.