In case of large templates and large content in the “output” panel, it is necessary to have scrollbars:
I managed to add these scrollbars by specifying CSS properties in Code Inspector:
Revised 29.07.22:
for the class=".cm-editor":
max-height: 70vh;
for the class="rendered string":
max-height: 77vh;
overflow-y: scroll;
added my +1
However, Feature requests here are for core, not necessarily for Frontend, which your FR is about. You’d best add it to Discussions · home-assistant/frontend · GitHub, where the frontend team checks for valuable suggestions.
Since you’ve already gone in depth on the necessary changes, let’s hope they’d pick this up