fix(server): 同一次输入内候选卡片只增不减

译文、云/AI 候选、混输英文等内容是陆续到达的，每一批都让卡片在两帧之间改变
宽高。每一帧本身画得完整，但尺寸来回变化看起来就是闪烁。

同一次组合内卡片只增大；自然尺寸低于保留尺寸的 70% 时（页面确实变小了）才跟着
收回，候选窗隐藏后重新开始。D2D 给 card 设置 MinWidth/MinHeight 后重新排版；
WebView2 在更新内容的同一个 JS 任务里给 #realContainer 和 #measureContainer
设置最小宽高，绘制前就是最终尺寸，原生区域测量的也是同一个盒子。两端的收回比例
常量互相注明了对应关系。

server/src/webview2/windows_webview2.cpp

@@ -1825,6 +1825,43 @@ html:not(.msime-hover-armed) #realContainer.hover-active .cand:not(.first):hover
 })();
 )";
 
+// Late additions to the page (translations, cloud/AI/English merges) resize the
+// card between two frames of one composition, which reads as flicker even
+// though each frame is complete. Within a composition the card only grows, and
+// shrinks back only when its natural size drops below kStickyShrinkRatio of the
+// kept one (the page really got smaller). Runs in the same task as the content
+// update, so the frame is laid out once with the final size; the native region
+// measures the same box and follows. An empty update (hide) ends the
+// composition. Mirrors kStickyCardShrinkRatio in the D2D candidate presenter.
+constexpr wchar_t kStickyCandidateCardScript[] = LR"(
+window.MsimeStickyCandidateCard = function (reset) {
+  const kStickyShrinkRatio = 0.7;
+  const box = document.getElementById('realContainer');
+  if (!box) return;
+  const measure = document.getElementById('measureContainer');
+  const boxes = measure ? [box, measure] : [box];
+  boxes.forEach(function (b) { b.style.minWidth = ''; b.style.minHeight = ''; });
+  if (reset) { window.__msimeStickyCard = null; return; }
+  box.style.boxSizing = 'border-box';
+  const rect = box.getBoundingClientRect();
+  const kept = window.__msimeStickyCard || {width: 0, height: 0};
+  const follow = function (keptValue, natural) {
+    return natural >= keptValue || natural < keptValue * kStickyShrinkRatio ? natural : keptValue;
+  };
+  let width = follow(kept.width, rect.width);
+  const maxWidth = Number.parseFloat(box.style.maxWidth);
+  if (maxWidth > 0) width = Math.min(width, maxWidth);
+  const height = follow(kept.height, rect.height);
+  window.__msimeStickyCard = {width: width, height: height};
+  if (width > rect.width + 0.5 || height > rect.height + 0.5) {
+    boxes.forEach(function (b) {
+      b.style.minWidth = width + 'px';
+      b.style.minHeight = height + 'px';
+    });
+  }
+};
+)";
+
 VOID CALLBACK CandidateHoverArmTimerProc(HWND, UINT, UINT_PTR, DWORD)
 {
     MaybeArmCandidatePointerHover();
@@ -2163,6 +2200,9 @@ void UpdateHtmlContentWithJavaScript(ComPtr<ICoreWebView2> webview, const std::w
     script.append(GetConfiguredCandidateWindowPreeditStyle() == "empty" ? L"false" : L"true");
     script.append(L"); }\n");
     script.append(L"if (window.SetPreeditCaret) { window.SetPreeditCaret(); }\n");
+    script.append(kStickyCandidateCardScript);
+    script.append(newContent.empty() ? L"window.MsimeStickyCandidateCard(true);\n"
+                                     : L"window.MsimeStickyCandidateCard(false);\n");
     script.append(L"if (window.CheckContentTruncation) { window.CheckContentTruncation(); }\n");
     if (diagnosticsEnabled)
     {

server/src/window/candidate_presenter.cpp

@@ -156,6 +156,16 @@ constexpr float kShadowPadRight = 32.0f;
 constexpr float kShadowPadBottom = 40.0f;
 constexpr float kCandidateMinWidthDip = 160.0f;
 constexpr float kDecorationCardOverlap = 5.0f;
+// Below this fraction of the kept extent, the page really got smaller (fewer or
+// shorter candidates) and the card follows it; above it, the difference is a
+// late addition coming or going and the card keeps its size. Mirrors
+// kStickyShrinkRatio in the WebView2 candidate script.
+constexpr float kStickyCardShrinkRatio = 0.7f;
+
+float StickyCardExtent(float kept, float natural)
+{
+    return natural >= kept || natural < kept * kStickyCardShrinkRatio ? natural : kept;
+}
 
 } // namespace
 
@@ -1076,12 +1086,41 @@ void CandidatePresenter::ShowFromGlobalState(POINT caret)
     const HalfScreenDipLimits limits = ApplyScaleToHalfScreenLimits(caret, scale.scale);
     const float maxW = limits.maxWidthDip > 1.0 ? static_cast<float>(limits.maxWidthDip) : 480.0f;
     const float maxH = limits.maxHeightDip > 1.0 ? static_cast<float>(limits.maxHeightDip) : 640.0f;
-    impl_->root->InvalidateMeasure();
-    const msimeui::SizeF measured = impl_->root->MeasureInLayout({maxW, maxH});
-    float widthDip = (std::max)(measured.width, kCandidateMinWidthDip + kShadowPadLeft + kShadowPadRight);
-    float heightDip = (std::max)(measured.height, 36.0f);
-    impl_->root->InvalidateArrange();
-    impl_->root->ArrangeInLayout({0.0f, 0.0f, widthDip, heightDip});
+    float widthDip = 0.0f;
+    float heightDip = 0.0f;
+    auto layoutRoot = [&]() {
+        impl_->root->InvalidateMeasure();
+        const msimeui::SizeF measured = impl_->root->MeasureInLayout({maxW, maxH});
+        widthDip = (std::max)(measured.width, kCandidateMinWidthDip + kShadowPadLeft + kShadowPadRight);
+        heightDip = (std::max)(measured.height, 36.0f);
+        impl_->root->InvalidateArrange();
+        impl_->root->ArrangeInLayout({0.0f, 0.0f, widthDip, heightDip});
+    };
+    // Late additions to the page (translations, cloud/AI/English merges) change
+    // the card's size between two frames of the same composition, and the eye
+    // reads that as flicker even though every frame is complete. Within one
+    // composition the card therefore only grows; it shrinks back only when the
+    // natural size falls well below the kept one (a page that really got
+    // smaller). Hide() starts the next composition from the natural size.
+    if (impl_->card)
+    {
+        impl_->card->SetMinWidth(kCandidateMinWidthDip);
+        impl_->card->ClearMinHeight();
+    }
+    layoutRoot();
+    if (impl_->card)
+    {
+        const msimeui::RectF natural = impl_->card->GetBounds();
+        const float maxCardWidthDip = (std::max)(kCandidateMinWidthDip, maxW - kShadowPadLeft - kShadowPadRight);
+        stickyCardWidthDip_ = (std::min)(StickyCardExtent(stickyCardWidthDip_, natural.width), maxCardWidthDip);
+        stickyCardHeightDip_ = StickyCardExtent(stickyCardHeightDip_, natural.height);
+        if (stickyCardWidthDip_ > natural.width + 0.5f || stickyCardHeightDip_ > natural.height + 0.5f)
+        {
+            impl_->card->SetMinWidth((std::max)(kCandidateMinWidthDip, stickyCardWidthDip_));
+            impl_->card->SetMinHeight(stickyCardHeightDip_);
+            layoutRoot();
+        }
+    }
     hoverArmed_ = false;
     if (!GetCursorPos(&hoverBaseline_))
     {
@@ -1122,6 +1161,8 @@ void CandidatePresenter::Hide()
     SetCandidateHostCloaked(true);
     lastHostWidthPx_ = 0;
     lastHostHeightPx_ = 0;
+    stickyCardWidthDip_ = 0.0f;
+    stickyCardHeightDip_ = 0.0f;
     SetWindowPos(hwnd_, nullptr, 0, Global::INVALID_Y, 0, 0,
                  SWP_NOSIZE | SWP_NOZORDER | SWP_NOACTIVATE | SWP_SHOWWINDOW);
 }

server/src/window/candidate_presenter.h

@@ -59,5 +59,8 @@ class CandidatePresenter
     int lastHostHeightPx_ = 0;
     float lastLayoutWidthDip_ = 0.0f;
     float lastLayoutHeightDip_ = 0.0f;
+    // Largest card size kept for the current composition; reset by Hide().
+    float stickyCardWidthDip_ = 0.0f;
+    float stickyCardHeightDip_ = 0.0f;
     int wheelDeltaAccumulator_ = 0;
 };