fix jumping inputbox's height

src/components/post_status_form/post_status_form.js

@@ -239,14 +239,12 @@ const PostStatusForm = {
       e.dataTransfer.dropEffect = 'copy'
     },
     resize (e) {
-      const target = e.target || e
-      target.style.height = 'auto'
-      const heightPx = target.scrollHeight - 10
-      if (heightPx > 54) {
-        target.style.height = `${target.scrollHeight - 10}px`
-      }
-      if (target.value === '') {
-        target.style.height = '16px'
+      const vertPadding = window.getComputedStyle(e.target)['padding-top'] +
+            window.getComputedStyle(e.target)['padding-bottom']
+      e.target.style.height = 'auto'
+      e.target.style.height = `${e.target.scrollHeight - vertPadding}px`
+      if (e.target.value === '') {
+        e.target.style.height = '16px'
       }
     },
     clearError () {