fix(web): keep ineligible message forks disabled

.agents/notes/implemented/bug-fix/2026-08-02-message-fork-actions-require-completed-turn-tail.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write .agents/notes/implemented/bug-fix/2026-08-02-message-fork-actions-require-completed-turn-tail.md
-2026-08-02-message-fork-actions-require-completed-turn-tail.md: 38a81d9e6966b459ed8d7dd19ba0cbdd790b85b6
-2026-08-02-message-fork-actions-require-completed-turn-tail.zh.md: 09d2ecc2b110e66348b7cd7c5eb253b4c8cdbb9a
+2026-08-02-message-fork-actions-require-completed-turn-tail.md: f2e7fd67b65a6ce4a86ba3f4405f78842be8f234
+2026-08-02-message-fork-actions-require-completed-turn-tail.zh.md: 2c3feeaa3ef01dbde67faa73257520918996f9c8

.agents/notes/implemented/bug-fix/2026-08-02-message-fork-actions-require-completed-turn-tail.md

@@ -10,7 +10,7 @@ The Web conversation attached branch to the last assistant node with nonempty te
 
 ## Decision
 
-`ConversationSnapshot.turnEnds` retains the completed turn boundaries present in the raw event window. The conversation view walks transcript nodes through each boundary and exposes branch only when the boundary's last node is a user message, a durable steering message, or a content-bearing assistant message. Open turns have no eligible message, and a later tool result, reasoning-only interruption, turn error, or other transcript node suppresses branch on earlier messages. Copy and clock remain available under their existing message chrome, and the Host's completed-turn fork semantics remain unchanged.
+`ConversationSnapshot.turnEnds` retains the completed turn boundaries present in the raw event window. The conversation view walks transcript nodes through each boundary and enables branch only when the boundary's last node is a user message, a durable steering message, or a content-bearing assistant message. Open turns have no eligible message, and a later tool result, reasoning-only interruption, turn error, or other transcript node leaves branch unavailable on earlier messages. The unavailable control stays visible, focusable, and hoverable; `aria-disabled`, a tooltip, and `aria-describedby` explain the completed-tail requirement without sending a Host request. Copy and clock remain available under their existing message chrome, and the Host's completed-turn fork semantics remain unchanged.
 
 This narrows the message eligibility established by the earlier [Web session fork action decision](../feature/2026-07-27-web-session-fork-actions.md). Session-row forking still selects the latest completed turn, and eligible message actions still pass their event seq through the shared client runtime operation.
 
@@ -22,6 +22,8 @@ This narrows the message eligibility established by the earlier [Web session for
 
 **Hide branch from every interrupted turn.** Rejected because an aborted turn is durably closed and its final interrupted text can be the true transcript tail. Eligibility depends on the completed boundary and node order, not the outcome kind.
 
+**Hide ineligible message controls.** Rejected because a disappearing control does not explain the boundary requirement and shifts otherwise stable message chrome. A focusable unavailable control preserves the affordance while preventing the request.
+
 ## Consequences
 
-A branch icon now denotes the same completed-turn boundary that the Host will copy. In the reported response → tool → interrupted Think shape, the response keeps copy and clock but no longer advertises branch. This change deliberately does not provide same-turn transcript editing or a retry-before-turn operation; the Session-row action remains available when a reader wants to copy the latest completed turn in full. Runtime tests pin boundary projection and reference stability, while conversation tests cover assistant, user-only, and durable-steering tails plus suppression by later tool and interrupted reasoning rows.
+An enabled branch icon denotes the same completed-turn boundary that the Host will copy. In the reported response → tool → interrupted Think shape, the response keeps copy, clock, and a disabled branch control that explains why it cannot act. This change deliberately does not provide same-turn transcript editing or a retry-before-turn operation; the Session-row action remains available when a reader wants to copy the latest completed turn in full. Runtime tests pin boundary projection and reference stability, while conversation tests cover assistant, user-only, and durable-steering tails plus unavailable controls caused by later tool and interrupted reasoning rows.

.agents/notes/implemented/bug-fix/2026-08-02-message-fork-actions-require-completed-turn-tail.zh.md

@@ -10,7 +10,7 @@ Web 会话把分支操作挂到每个轮次中最后一个文本非空的 assist
 
 ## 决策
 
-`ConversationSnapshot.turnEnds` 保留原始事件窗口中的已完成轮次边界。会话视图按各边界遍历 transcript（文本记录）节点，仅当边界的最后一个节点是用户消息、持久 steering（中途引导）消息或含内容的 assistant 消息时才暴露分支操作。开放轮次没有符合条件的消息；如果后面还有工具结果、只有推理内容的中断、轮次错误或其他 transcript 节点，较早消息上的分支操作就会被抑制。复制和时钟仍可在既有消息 chrome 下使用，Host 按已完成轮次 fork 的语义保持不变。
+`ConversationSnapshot.turnEnds` 保留原始事件窗口中的已完成轮次边界。会话视图按各边界遍历 transcript（文本记录）节点，仅当边界的最后一个节点是用户消息、持久 steering（中途引导）消息或含内容的 assistant 消息时才启用分支操作。开放轮次没有符合条件的消息；如果后面还有工具结果、只有推理内容的中断、轮次错误或其他 transcript 节点，较早消息上的分支操作会保持不可用。不可用的控件仍然可见、可聚焦、可悬停；`aria-disabled`、tooltip 与 `aria-describedby` 会说明已完成尾部这一要求，且不会发送 Host 请求。复制和时钟仍可在既有消息 chrome 下使用，Host 按已完成轮次 fork 的语义保持不变。
 
 本决策收紧了较早的 [Web 会话 fork 操作决策](../feature/2026-07-27-web-session-fork-actions.md)所定义的消息资格。Session 行 fork 仍选择最新的已完成轮次；符合条件的消息操作仍通过共享 client 运行时操作传递其事件 seq。
 
@@ -22,6 +22,8 @@ Web 会话把分支操作挂到每个轮次中最后一个文本非空的 assist
 
 **对每个被中断轮次隐藏分支。** 不予采纳：已中止的轮次会持久关闭，其最终的中断文本可能正是真正的 transcript 尾部。资格取决于已完成边界与节点顺序，而非结果类别。
 
+**隐藏不符合条件的消息控件。** 不予采纳：消失的控件无法说明边界要求，还会让本应稳定的消息 chrome 发生位移。保留可聚焦但不可用的控件，既能维持操作提示，也能阻止请求。
+
 ## 后果
 
-分支图标现在表示的已完成轮次边界与 Host 实际复制的边界一致。在所报告的「响应 → 工具 → 被中断的 Think」形态中，响应仍保留复制和时钟，但不再显示分支。本变更刻意不提供同轮次 transcript 编辑，也不提供轮次前重试操作；当读者希望完整复制最新的已完成轮次时，仍可使用 Session 行操作。运行时测试固定边界投影和引用稳定性，会话测试则覆盖 assistant 尾部、纯用户消息尾部、持久 steering 尾部，以及后续工具行和被中断推理行对分支操作的抑制。
+启用的分支图标现在表示的已完成轮次边界与 Host 实际复制的边界一致。在所报告的「响应 → 工具 → 被中断的 Think」形态中，响应仍保留复制、时钟，以及一个说明无法操作原因的禁用分支控件。本变更刻意不提供同轮次 transcript 编辑，也不提供轮次前重试操作；当读者希望完整复制最新的已完成轮次时，仍可使用 Session 行操作。运行时测试固定边界投影和引用稳定性，会话测试则覆盖 assistant 尾部、纯用户消息尾部、持久 steering 尾部，以及后续工具行和被中断推理行导致的不可用控件。