fix #371: straddling rows clip at the viewport edge instead of vanishing whole

Gate-3 screenshot review (user): 'the chat tab looks like it is missing
per window config' — Chat Window 1's header rendered over a void at the
DEFAULT scroll offset because its 260px self-sized filter block
straddled the viewport's bottom edge and UiScrollablePanel hid
straddling rows WHOLE (AP-201's predicted symptom, now user-observed at
scroll position zero, upgrading it from polish to blocking).

By fix time the UI renderer HAD everything needed: UiRenderContext's
clip stack (PushClip/PopClip with rect intersection + per-draw quad
clipping) and UiElement's ClipsChildren hook, already honored by both
the generic draw walk and hit-testing. The fix is therefore exactly the
shape the filing asked for, in the panel itself:
- ClipsChildren => true: children draw and hit-test clipped to the
  viewport rect.
- The layout cull keeps any INTERSECTING row Visible (was: fully-inside
  only), with a half-pixel margin excluding zero-overlap edge rows;
  fully-outside rows stay hidden as the cheap skip.

AP-201 retired in this commit (AP actives 142 -> 141); #371 closed; the
gate script's Chat-tab steps re-written to expect clean edge clipping
and to treat any whole-block vanish as a regression. Pinned by
StraddlingRow_StaysVisible_AndClipsInsteadOfVanishing (the exact gate-3
geometry: header + 260px straddler in a 430px viewport) and
ViewportClipsChildDrawingAndHitTesting (the clipped slice is not
clickable).

Full Release suite: 13,089 passed / 4 skipped / 0 failed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/research/2026-08-11-campaign-op-test-script.md

@@ -403,16 +403,15 @@ chat windows already read when deciding which lines to show.
    block has all 13.
 2. **Scroll the list.** All six sections are reachable; the block heights
    visibly differ from a flat 100px box — each block is exactly tall enough
-   to show all of its own rows with no clipping and no dead space below the
-   last row (AP-195's self-sizing). **Known, registered behavior (AP-201,
-   OP5 review fix S2): the row viewport culls whole rows rather than
-   clipping them (no scissor stack yet — `UiScrollablePanel.cs`), so a
-   240-260px filter block that straddles the visible edge at a given scroll
-   position can vanish ENTIRELY at that exact offset rather than showing a
-   partially-clipped view. Keep scrolling a little further and the block
-   reappears whole. This is the registered cull-vs-clip divergence, not a
-   self-sizing regression — do not report a block's disappear-then-
-   reappear-whole behavior as a bug.**
+   to show all of its own rows with no dead space below the last row
+   (AP-195's self-sizing). **Gate-3 fix (#371, AP-201 retired): blocks now
+   CLIP at the viewport edges instead of vanishing whole.** At the default
+   scroll position you should see the Main Chat Window block in full, then
+   "Chat Window 1"'s header and as many of its checkboxes as fit, cut off
+   cleanly at the panel's bottom edge — scrolling reveals the rest
+   smoothly. A block that disappears ENTIRELY while its header stays (the
+   pre-fix void your gate-3 screenshot review caught) is a regression —
+   report it.
 3. **Look closely at a row whose mask covers MULTIPLE underlying message
    types** (Gameplay, Combat, Allegiance, or Fellowship — the composite-mask
    rows per the research doc) versus a single-bit row (e.g. "Error" or
@@ -518,10 +517,9 @@ chat windows already read when deciding which lines to show.
 - Reset/Defaults not behaving as described in 11-12.
 - Any block that is STILL missing rows, or leaves a large empty gap below
   its last row, once fully scrolled into view (a genuine self-sizing
-  regression). **Not a regression:** a block that disappears entirely at
-  some intermediate scroll position and reappears whole a little further —
-  that is the registered whole-row-cull behavior (AP-201, see step 2), not
-  a clipping bug.
+  regression). A block that disappears ENTIRELY at any scroll position is
+  ALSO a regression now — the gate-3 fix (#371) made straddling blocks
+  clip at the viewport edge instead of vanishing whole (see step 2).
 - Any setting that reverts to default after a full relaunch (a persistence
   regression) — remember this is local-only, so a SERVER-side relog is not
   the right test here (see item 15's note).