On lower (ex. 1024x768) resolutions (or when there's a lot of text) the $parameter buttons obstructs the bottom part of the edit box - they cover part of the text, and don't allow to set position of the cursor by clicking. Please move it outside of the edit field (ex. below it).
Placing below into your user skin css do that:
.grid .tux-editor-actions-block .tux-editor-insert-buttons{
position: initial;
}
Do you happen to use Firefox 88? There’s a ticket about this issue in Firefox 87–88, but older and newer versions (including Firefox 89, released as stable two days ago) have been reported to be unaffected. If you have the same issue, simply updating Firefox (hamburger menu → Help → About Firefox) should fix it.
Yes, I was using FF 88. After update the edit field expands correctly.
By the way, I noticed that the $param buttons shows after the documentation loads in the right panel (it can sometimes take a few seconds). If they are not connected (but they probably are), then maybe display those buttons before starting to load the doc?
They’re connected, the so-called “insertables” ($param buttons) are also computed on server-side, and loaded in the same HTTP request as the documentation (for the message on your screenshot, it’s https://www.mediawiki.org/w/api.php?action=translationaids&format=json&title=Translations:Help:Magic_words/8/pl). Although I don’t know whether they’re actually interconnected on the server side, or would it be possible to split up the API action.