Fix T78503: Disabled layout portion passes mouse through

3D Viewport -> Sidebar -> View -> Local Camera is disabled. When you
click on it, box select starts. This behavior isn't intended, and fixing
it is quite simple. The only case where this would continuing would
still be desired is if a disabled button overlapped with a non-disabled
button, but this shouldn't be the case anywhere.

Differential Revision: https://developer.blender.org/D8832

source/blender/editors/interface/interface_handlers.c

@@ -7580,7 +7580,7 @@ static int ui_do_button(bContext *C, uiBlock *block, uiBut *but, const wmEvent *
   }
 
   if (but->flag & UI_BUT_DISABLED) {
-    return WM_UI_HANDLER_CONTINUE;
+    return WM_UI_HANDLER_BREAK;
   }
 
   switch (but->type) {