[BUGFIX] Add space below "new element" button

The "create new element" button in the form
editor was cut off.

This is fixed by calculating the height
minus the doc-header height.

Resolves: #94773
Releases: master
Change-Id: I7dfc891138473592c76562c6802430a4311739a2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70394
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: waldhacker <hello@waldhacker.dev>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: waldhacker <hello@waldhacker.dev>

Build/Sources/Sass/form.scss

@@ -518,7 +518,8 @@ $tree-line-height: 20px;
 #t3-form-stage-container {
     overflow: auto;
     position: relative;
-    height: 100%;
+    height: calc(100% - #{$module-docheader-height});
+    max-height: 100%;
     text-align: center;
 
     ol,