This Meta SE Q&A suggests that there may be some hope: Allow questions to be saved as drafts prior to posting.
Hopefully, you will not be stymied by "Drafts are not supported on self-answer."
I cannot remember whether I've ever seen it work. I think I may have on occasion, but I also think that I have seen no draft available much more often. I tend to just use Notepad to write drafts of posts longer than I am willing to risk to my flaky browser/graphics card combination.
@AndyW has reported some success in some circumstances (but not others):
I know I've part-written an answer before, then navigated away, and on
returning found the draft still there. But I just tested it (only
wrote a couple of lines then left the page) and it didn't work. Tried
again, left it longer, and saw "draft saved" under the answer field,
and did recover the draft on return.