When composing a reply to a post on my iPhone (11, plain vanilla), the composition window is often obscured by the post that I’m responding to. Scrolling down to the end of the window is not helpful, as the system is resistant to allowing that. My browser is Firefox.
If you’re using firefox on your phone, the issue may be in your browser, perhaps some setting or other?
I use the default safari browser on my xr, briefly used the ios version of brave browser and have never experienced what you describe unless I’m trying to post a link, but that’s not unique to iphones, it did that to me when I was using android also.
In fact I sometimes find that I have to minimize the text window to re-read the post if it is long or complex enough.
We recently changed it so the editor is fullscreen on Android and iPhone, since people seemed to prefer this, and it is more consistent. Prior to this change, the editor was fullscreen on iOS but not on Android. Now it is fullscreen by default on both iOS and Android.
If you wish to collapse the mobile editor, so you can quote and read as you reply, press the chevron button at upper right.