bug#50424: 27.2; Tab bar button mouse face not clearing entirely

[Juri Linkov]

> If we want the default value of tab-bar-button-margin to be a cons
> cell, let's change the default value to be a cons cell, it's not more
> complicated than setting it to a scalar, even in C.  But tab-bar.el
> should use in its image specs the exact value of
> tab-bar-button-margin, it cannot decide to change it behind the back
> of the caller.

I tried a cons cell, but it looks ugly for buttons other than the close button.
OTOH, margin 1 looks nice for all buttons:

diff --git a/src/dispextern.h b/src/dispextern.h
index f4c7575b35..6aefe43e19 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -3202,7 +3202,7 @@ #define IMAGE_CACHE_BUCKETS_SIZE 1001
 
 /* Default values of the above variables.  */
 
-#define DEFAULT_TAB_BAR_BUTTON_MARGIN 4
+#define DEFAULT_TAB_BAR_BUTTON_MARGIN 1
 #define DEFAULT_TAB_BAR_BUTTON_RELIEF 1