I am posting my comments as an answer.

Let $k$ be a field. Let $$(G,m:G\times_{\text{Spec}\ k}G \to G)$$ be a locally finitely group scheme over $\text{Spec}\ k$. For every open $U$, denote by $m_U$ the restriction of $m$, $$m_U:U\times_{\text{Spec}\ k}U \to G.$$

Lemma 1. If $G$ is connected, then for every nonempty open $U$, the morphism $m_U$ is surjective. In particular, $G$ is quasi-compact.

Proof. The identity component of every group scheme is geometrically irreducible. Since $G$ is connected, it is geometrically irreducible. Thus, for an algebraically closed field extension $L/k$, for every $L$-point $g$ of $G$, the base change open $U_L\subset G_L$, its inverse $U_L^{-1}$, and its translate $g\cdot U_L^{-1}$ are each dense opens in $G_L$. Therefore, the intersection of $g\cdot U_L^{-1}$ and $U_L$ is nonempty. So there exists $g\cdot h^{-1}\in g\cdot U_L$ and $h'\in U_L$ that are equal, i.e., $g=h\cdot h'$ for $(h,h')\in U_L \times_{\text{Spec}\ L}U_L.$ QED

Lemma 2. For every flat, locally finitely presented morphism of schemes, every point of the domain has an open affine neighborhood that is fppf over an open affine neighborhood of the image point in the target.

Proof. Without loss of generality, assume that the target and the domain are affine. Thus the morphism corresponds to ring homomorphism, $$\phi:B\to A.$$ Since the morphism is flat and locally finitely presented, the image is open in $\text{Spec}(B)$. A basis for the topology on $\text{Spec}(B)$ consists of distinguished opens $\text{Spec}(B_f)$. Thus, the image open equals the union of all distinguished opens $\text{Spec}(B_f)$ that it contains. For each such, the following ring homomorphism gives an fppf morphism of schemes, $$\phi_f:B_f \to A_f.$$ QED

Let $S$ be a scheme. Let $$(\pi_S:G_S\to S, m_S:G_S\times_S G_S \to G_S, i_S:G_S\to G_S,e_S:S\to G_S),$$ be a flat, locally finitely presented group scheme over $S$.

Proposition 3. If the fibers of $\pi_S$ are connected, then $\pi_S$ is finitely presented.

Proof. It suffices to prove this locally on the target. For every point $s$ of $S$, for the image point $e_S(s)$, by Lemma 2 there exists an open affine neighborhood $V$ of $s$ in $S$ and an open affine neighborhood $U$ of $e_S(s)$ in $G_S$ such that the morphism $U\to V$ is fppf.

By Lemma 1, the following morphism is surjective on fibers over geometric points of $V$, $$m_U: U\times_V U \to V \times_S G_S.$$ Since $U$ and $V$ are affine, also $U\times_V U$ is affine. Thus, $U\times_V U$ is quasi-compact. Since $V\times_S G_S$ is the image of a surjective morphism from a quasi-compact scheme, also $V\times_S G_S$ is a quasi-compact scheme. QED

I am posting my comments as an answer.

Let $k$ be a field. Let $$(G,m:G\times_{\text{Spec}\ k}G \to G)$$ be a locally finitely presented group scheme over $\text{Spec}\ k$. For every open $U$, denote by $m_U$ the restriction of $m$, $$m_U:U\times_{\text{Spec}\ k}U \to G.$$

Lemma 1. If $G$ is connected, then for every nonempty open $U$, the morphism $m_U$ is surjective. In particular, $G$ is quasi-compact.

Proof. The identity component of every group scheme is geometrically irreducible. Since $G$ is connected, it is geometrically irreducible. Thus, for an algebraically closed field extension $L/k$, for every $L$-point $g$ of $G$, the base change open $U_L\subset G_L$, its inverse $U_L^{-1}$, and its translate $g\cdot U_L^{-1}$ are each dense opens in $G_L$. Therefore, the intersection of $g\cdot U_L^{-1}$ and $U_L$ is nonempty. So there exists $g\cdot h^{-1}\in g\cdot U_L$ and $h'\in U_L$ that are equal, i.e., $g=h\cdot h'$ for $(h,h')\in U_L \times_{\text{Spec}\ L}U_L.$ QED

Lemma 2. For every flat, locally finitely presented morphism of schemes, every point of the domain has an open affine neighborhood that is fppf over an open affine neighborhood of the image point in the target.

Proof. Without loss of generality, assume that the target and the domain are affine. Thus the morphism corresponds to ring homomorphism, $$\phi:B\to A.$$ Since the morphism is flat and locally finitely presented, the image is open in $\text{Spec}(B)$. A basis for the topology on $\text{Spec}(B)$ consists of distinguished opens $\text{Spec}(B_f)$. Thus, the image open equals the union of all distinguished opens $\text{Spec}(B_f)$ that it contains. For each such, the following ring homomorphism gives an fppf morphism of schemes, $$\phi_f:B_f \to A_f.$$ QED

Let $S$ be a scheme. Let $$(\pi_S:G_S\to S, m_S:G_S\times_S G_S \to G_S, i_S:G_S\to G_S,e_S:S\to G_S),$$ be a flat, locally finitely presented group scheme over $S$.

Proposition 3. If the fibers of $\pi_S$ are connected, then $\pi_S$ is finitely presented.

Proof. It suffices to prove this locally on the target. For every point $s$ of $S$, for the image point $e_S(s)$, by Lemma 2 there exists an open affine neighborhood $V$ of $s$ in $S$ and an open affine neighborhood $U$ of $e_S(s)$ in $G_S$ such that the morphism $U\to V$ is fppf.

By Lemma 1, the following morphism is surjective on fibers over geometric points of $V$, $$m_U: U\times_V U \to V \times_S G_S.$$ Since $U$ and $V$ are affine, also $U\times_V U$ is affine. Thus, $U\times_V U$ is quasi-compact. Since $V\times_S G_S$ is the image of a surjective morphism from a quasi-compact scheme, also $V\times_S G_S$ is a quasi-compact scheme. QED