There's no reason this couldn't be added if someone wanted to create a clean pull request and prove that it won't have a negative performance impact, even on huge documents.
Browsers automatically do a progressive find all, they don't wait for you to type the search pattern then return the first result so their behaviour is irrelevant.
I assume the OP is wanting Geany to behave more like browsers/other editors, so it's not really irrelevant.
A couple examples:
![atom_search](https://user-images.githubusercontent.com/181177/58149527-36aede80-7c18-11e9...)
![firefox_search](https://user-images.githubusercontent.com/181177/58149535-3dd5ec80-7c18-11e9...)