When I create a new branch and PR stacked on top of an existing, unmerged bra...

When I create a new branch and PR stacked on top of an existing, unmerged branch, the session shows the PR number of the base branch's PR instead of the newly-created branch and PR. The Changes view contains both PRs' diffs in a single view (i.e. the new branch diff'd against main).

The new session should correclty show the PR and diffs of the new, stacked branch compared to the base, existing branch.

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| Field | Value |
| --- | --- |
| App version | 1.1.12 |
| OS | macOS 26.6.2 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 2 |

未指定任何文件或测试。首先在 /chat 会话中复现该流程：创建一个叠加在现有未合并分支之上的分支和 PR，然后检查显示的 PR 编号和 Changes 视图。完成的标准是新会话显示新的 PR，并且只显示其相对于现有基分支的 diff。

由索引模型根据 Issue 内容生成。