The Stacks project

Proof. If $f$ is of finite presentation, then this follows immediately from Lemmas 76.8.5 and 76.3.6. This is the only case of interest and we urge the reader to skip the rest of the proof, which deals with the possibility (allowed by the assumptions in this lemma) that $f$ is not quasi-separated or quasi-compact.

Let $i : Z \to X$ be the closed subspace cut out by the zeroth fitting ideal of $\mathcal{G}$ (Divisors on Spaces, Section 70.5). Then $Z \to B$ is proper by assumption (see Derived Categories of Spaces, Section 74.7). On the other hand $i$ is of finite presentation (Divisors on Spaces, Lemma 70.5.2 and Morphisms of Spaces, Lemma 66.28.12). There exists a quasi-coherent $\mathcal{O}_ Z$-module $\mathcal{H}$ of finite type with $i_*\mathcal{H} = \mathcal{G}$ (Divisors on Spaces, Lemma 70.5.3). In fact $\mathcal{H}$ is of finite presentation as an $\mathcal{O}_ Z$-module by Algebra, Lemma 10.6.4 (details omitted). Then $F_{zero}$ is the same as the functor $F_{zero}$ for the map $i^*\mathcal{F} \to \mathcal{H}$ adjoint to $u$, see Lemma 76.7.6. The sheaf $\mathcal{H}$ is flat relative to $B$ because the same is true for $\mathcal{G}$ (check on stalks; details omitted). Moreover, note that if $\mathcal{F}$ is of finite type, then $i^*\mathcal{F}$ is of finite type. Hence we have reduced the lemma to the case discussed in the first paragraph of the proof. $\square$