When an ISTJ is jealous, it may not be immediately apparent as they tend to keep their emotions and reactions under control. However, there are certain signs and behaviors that may indicate their jealousy. Here is a detailed description of how an ISTJ may behave when they are feeling jealous:

1. Increased Observational Behavior: ISTJs are naturally observant individuals, and when they are jealous, they may become even more attentive to their partner's actions. They may closely monitor their partner's interactions with others, paying attention to any signs of potential threats or competition.

2. Possessiveness: ISTJs value loyalty and commitment in relationships, and when they feel jealous, they may become possessive of their partner. They may want to spend more time with their partner, become more protective, and may even express a desire to control their partner's actions or choices.

3. Withdrawal: ISTJs are introverted individuals who often need time alone to process their emotions. When they are feeling jealous, they may withdraw from social situations or become more reserved in their interactions. They may need space to reflect on their feelings and figure out how to address their jealousy.

4. Increased Attention to Detail: ISTJs are known for their attention to detail and practicality. When they are jealous, they may start noticing small changes in their partner's behavior or appearance. They may become hyper-aware of any potential signs of infidelity or disloyalty, even if they are not necessarily present.

5. Overthinking and Analysis: ISTJs have a tendency to overthink and analyze situations, and this trait becomes more pronounced when they are feeling jealous. They may constantly replay scenarios in their mind, trying to find evidence to support or dismiss their feelings of jealousy. This can lead to a cycle of rumination and increased anxiety.

6. Emotional Restraint: ISTJs are not typically known for expressing their emotions openly. When they are jealous, they may try to suppress their feelings and maintain a composed exterior. They may struggle to communicate their jealousy directly, which can lead to internalized frustration and resentment.

7. Increased Need for Reassurance: Despite their stoic nature, ISTJs still have a need for reassurance and validation in their relationships. When they are feeling jealous, they may seek reassurance from their partner, wanting to hear affirmations of their commitment and loyalty. They may also appreciate gestures of affection and attention to help alleviate their feelings of jealousy.

It's important to note that jealousy is a complex emotion, and individuals may experience it differently based on their personal experiences and beliefs. While this description provides a general understanding of how an ISTJ may behave when jealous, it's essential to remember that everyone is unique, and individual differences should be considered.