docs(vt): document that plain <menu> style covers the open popup too

Follow-up to the UiMenu popup fix: the existing "menu style" paragraph in
plugin-ui-markup.md only described the closed-state button face swap from
the earlier S7 fix. Extend it to say the plain style now covers the whole
menu (closed AND open) — flat popup chrome matching <list>, a lighter hover
fill, no checkmark, and a plain scrollbar past the row cap — so a plugin
author reading the doc doesn't assume style="plain" only affects the
closed face.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/plugin-ui-markup.md

@@ -117,7 +117,18 @@ list boxes (owner live-client report, 2026-09-07), so a plugin `<menu>` now
 draws the flat VTank/Decal `HudCombo` box (list-matching fill/border, a
 left-aligned value, and a small ▾) by default; `style="retail"` opts back
 into the gold face for a panel that genuinely wants it. Any other value
-throws `FormatException` at `Build`.
+throws `FormatException` at `Build`. The plain style covers the WHOLE menu,
+closed and open: a follow-up owner report (still 2026-09-07 — "Drop down
+menus look horrible, there is also a checkmark on the text there") found the
+OPEN popup still drew retail's tan/orange gradient panel, its ornate gold
+scrollbar, and a baked checkmark glyph on the current entry even with
+`style="plain"`. The open popup now matches `<list>`'s own chrome too: a
+flat fill + 1px border, one row per entry in the list text color, the
+current entry filled like a list selection, the hovered entry a slightly
+lighter fill, and no checkmark; more entries than the row cap show a plain
+1px-bordered scrollbar track with a flat thumb, no DAT scrollbar art.
+`style="retail"` keeps the sprite popup (gradient panel, checkmark-bearing
+row art, ornate scrollbar) exactly as before, unchanged.
 
 Common to every element via `ApplyCommon`: `name`/`id` (a stable control
 name), `visible` (literal `true`/`false` or a bound `bool` property),