patch 8.0.0110
Problem:    Drop command doesn't use existing window.
Solution:   Check the window width properly. (Hirohito Higashi)
diff --git a/src/testdir/test_tabpage.vim b/src/testdir/test_tabpage.vim
index 3f69fb9..1720107 100644
--- a/src/testdir/test_tabpage.vim
+++ b/src/testdir/test_tabpage.vim
@@ -65,6 +65,15 @@
     call assert_true(tabpagenr() == 2 && tabpagewinnr(2, '$') == 2 && tabpagewinnr(2) == 1)
     tabclose
     q
+    "
+    "
+    " Test for ":tab drop vertical-split-window" to jump test1 buffer
+    tabedit test1
+    vnew
+    tabfirst
+    tab drop test1
+    call assert_equal([2, 2, 2, 2], [tabpagenr('$'), tabpagenr(), tabpagewinnr(2, '$'), tabpagewinnr(2)])
+    1tabonly
   endif
   "
   "