Merge remote-tracking branch 'origin/master' into worktree/context-source-cards

# Conflicts:
#	packages/client/runtime/README.i18n.yaml
#	packages/client/runtime/README.md
#	packages/client/runtime/README.zh.md
#	packages/client/ui-conversation/README.i18n.yaml

packages/client/ui-conversation/src/client/contract/diff-card-model.ts

@@ -76,11 +76,8 @@ function narrowDiffs(diffs: unknown): DiffHunk[] | null {
  *
  * This derivation consumes only `diffs`; the render intent's `title` field is
  * deliberately dropped. The row supplies its own title (`Edit`/`Write · path`
- * from the args) and that outranks the view's `title`, matching the TUI diff
- * branch, which likewise draws no view title. A tool that names its own diff
- * header therefore does not surface that text on the Web row — an accepted
- * product choice, recorded here as the one asymmetry with the terminal card,
- * whose derivation does consume the view's title.
+ * from the args), which outranks the view's `title`. A tool that names its own
+ * diff header therefore does not surface that text on the Web row.
  * @param block - RunningToolCall or ToolResultNode off the snapshot caches.
  * @returns the diff-card props, or null for the generic path.
  */

packages/client/ui-conversation/src/client/skeleton/ConversationRoot.module.css

@@ -191,6 +191,11 @@
   flex-direction: column;
   min-height: 0;
   overflow-y: auto;
+  /* Reserved unconditionally: the composer seat rides this box's content box in
+     Chat and its padding box under a view's composer overlay, so an `auto`
+     gutter moves the input card sideways by the bar's width whenever the two
+     differ ([decision](../../../../../../.agents/notes/implemented/bug-fix/2026-08-04-composer-tab-gutter-reservation.md)). */
+  scrollbar-gutter: stable;
 }
 
 .root[data-phase='active'] .viewArea {
@@ -221,7 +226,13 @@
    ownership of the seat geometry and its active-phase precedence. */
 .scrollBody:has([data-conversation-composer-overlay]) {
   position: relative;
-  overflow: hidden;
+  /* A clipping box nothing scrolls out of, stated as a scroll container on both
+     axes rather than `overflow: hidden`: WebKit honours the reservation above
+     only in the `overflow-y: auto` form, and a single-axis scroller computes
+     the other axis to `auto`
+     ([decision](../../../../../../.agents/notes/implemented/bug-fix/2026-08-04-composer-tab-gutter-reservation.md)). */
+  overflow-x: hidden;
+  overflow-y: auto;
 }
 
 .scrollBody:has([data-conversation-composer-overlay]) > .viewArea {