fix(web/style): 鼠标 hover post-list 时滚动条占位导致内容左右抖动

CHANGELOG.md

@@ -27,6 +27,7 @@ auto-generated notes — but stable semver tags must pass.
 ## Unreleased
 
 - Thread view no longer flickers on every refresh. The post list is now diffed in place (keyed by post id), so unchanged posts keep their existing DOM nodes — SSE bursts, the 8s poll fallback, and cross-tab broadcasts can all fire together without repainting the column. When SSE is healthy the per-thread poll is suppressed entirely.
+- Post column no longer shifts left/right when the pointer hovers the thread view. The Slack-style hover-reveal scrollbar now reserves its gutter at all times, so toggling the bar in/out doesn't reflow the column.
 - _Add entries here as they ship. They get cut into the next `v<semver>` section at release time._
 
 <!--

web/style.css

@@ -1382,6 +1382,12 @@ body.squad-collapsed #squad-list::-webkit-scrollbar { display: none; }  /* WebKi
   /* Hide scrollbar chrome by default — cleaner reading surface; bar
      reappears on :focus-within for keyboard users (a11y). */
   scrollbar-width: none;
+  /* Always reserve the scrollbar gutter on the right so the Slack-style
+     hover-reveal scrollbar below doesn't reflow the post column when the
+     pointer enters/leaves. Without this, toggling scrollbar-width from
+     `none` to `auto` on :hover physically pushes content ~10px left, then
+     back — the user-reported "闪" when the cursor moves over post-list. */
+  scrollbar-gutter: stable;
 }
 #post-list::-webkit-scrollbar { display: none; }
 /* Show scrollbar when the user hovers the messages pane (Slack-style: bar