something to do with the modal-body class it seems

maybe this one? https://github.com/joomla/joomla-cms/pull/9817/files

@andrepereiradasilva and @brianteeman i've got it!
This issue is not link directly to this, as the issue is due to a wrong selector for modal script: $selector Unique DOM identifier for the modal. CSS id without #.

There's a quick fix for this issue with template edit modals: adding over-flow:hidden to .modal-body class, but this not fix overflow for the modal... I'm searching now where the modal id is set for both buttons folder and file, as modal selector in DOM is isisModal for both, but should be unique : folderModal and fileModal.

So, the srolling script fails as not the good id set as selector...

I'm currently searching for a fix for the template modals ;-)

I've opened a PR only to fix the display issue: #9897

About the $selector i think there's another issue, as the modal layout always get the same id for each modal ($template-modal) when it should be unique for each modal: collapseModal, folderModal, fileModal, ...

So, the display will be ok, but script used in modal main layout will not have any effect due to the wrong selector DOM identifier for each modal...
Note: i've found this not related issue to the current issue on display, only in the template edition page... (maybe to do the way com_templates manage the modals for buttons... all in the same files, and not in layouts ?...)

Opened a PR (and close #9897) to fix com_templates PR, as well as display issue.
Now each modal in com_templates template view use JLayouts ;-)
#9908

Closed as we have a PR for testing - thank @joomlic