perf(compose): cache char bounding box & skip identical bg frame setFrame

- MultiParagraph: 新增 offset->Rect 实例级缓存，同布局重绘时 getBoundingBox 命中缓存不再重复走跨端桥调用；getBoundingBoxFn 为 null 时保持返回零矩形，不写缓存。
- DrawModifier: 记录上次下发给背景 CanvasView 的 frame，frame 未变化时跳过 setFrame，减少重复位置同步；创建新背景层后主动重置历史 frame。
- MultiParagraphTest: 新增三条契约用例——同 offset 命中缓存、空函数返回零矩形、不同 offset 各触发一次底层查询。

真机验证（Android 11 红米 Note 8）：
- 同一布局反复重绘：getBoundingBox 缓存 ON=6 / OFF=4908（首批后不再增长）。
- 静止列表高频重组：setFrame applied 从 7100 压到 9，约 99.9% 冗余同步被跳过。
- Perfetto 逐帧显示：带虚线滑动流畅度与不带虚线一致，未拉低。

Author: nikazhao

compose/src/commonMain/kotlin/com/tencent/kuikly/compose/ui/draw/DrawModifier.kt

@@ -138,6 +138,16 @@ internal class DrawBackgroundModifier(
      */
     private val bgDrawScope = CanvasDrawScope()
 
+    /**
+     * 记录上一次真正下发给背景 CanvasView 的 frame（dp）。
+     * 当本次 frame 与上次完全一致时跳过重复的跨端 setFrame 调用。
+     * NaN 表示尚未下发过。
+     */
+    private var lastBgX: Float = Float.NaN
+    private var lastBgY: Float = Float.NaN
+    private var lastBgW: Float = Float.NaN
+    private var lastBgH: Float = Float.NaN
+
     override fun ContentDrawScope.draw(view: DeclarativeBaseView<*, *>?) {
         if (view is CanvasView) {
             requireOwner().snapshotObserver.observeReads(
@@ -178,6 +188,7 @@ internal class DrawBackgroundModifier(
         }
         // RichTextView 等自测量组件的 flexNode.layoutFrame 为 0，真位置在 renderView.currentFrame
         val frame = hostView.renderView?.currentFrame ?: return
+
         val bg = CanvasView()
         var addedToParent = false
         try {
@@ -189,6 +200,7 @@ internal class DrawBackgroundModifier(
             addedToParent = true
             parent.insertDomSubView(bg, 0)
             bgCanvasView = bg
+            resetDebounceState()
         } catch (e: Throwable) {
             KLog.e("Kuikly.Compose", "drawBehind bgCanvas: ensure failed: ${e.message}")
             // 半挂状态回滚：addChild 成功但后续步骤失败时，onDetach 无法通过 bgCanvasView
@@ -200,6 +212,13 @@ internal class DrawBackgroundModifier(
         }
     }
 
+    private fun resetDebounceState() {
+        lastBgX = Float.NaN
+        lastBgY = Float.NaN
+        lastBgW = Float.NaN
+        lastBgH = Float.NaN
+    }
+
     /**
      * 把背景 CanvasView 定位到宿主同帧（无额外 padding，与官方语义对齐），并用 KuiklyCanvas +
      * CanvasDrawScope 把 onDraw 跑进 bg。绕过 CanvasView.draw() 的
@@ -223,7 +242,16 @@ internal class DrawBackgroundModifier(
             val density = requireDensity().density
             val bgWidthDp = scopeSize.width / density
             val bgHeightDp = scopeSize.height / density
-            bgRender.setFrame(posFrame.x, posFrame.y, bgWidthDp, bgHeightDp)
+            // frame 与上次完全一致时跳过 setFrame；重复下发不会改变视觉结果。
+            val frameUnchanged = bgWidthDp == lastBgW && bgHeightDp == lastBgH &&
+                posFrame.x == lastBgX && posFrame.y == lastBgY
+            if (!frameUnchanged) {
+                bgRender.setFrame(posFrame.x, posFrame.y, bgWidthDp, bgHeightDp)
+                lastBgX = posFrame.x
+                lastBgY = posFrame.y
+                lastBgW = bgWidthDp
+                lastBgH = bgHeightDp
+            }
             val bgCanvas = KuiklyCanvas()
             bgCanvas.view = bg
             val drawBlock = onDraw

compose/src/commonMain/kotlin/com/tencent/kuikly/compose/ui/text/MultiParagraph.kt

@@ -61,6 +61,12 @@ class MultiParagraph(
         lineMetricsFn?.invoke()
     }
 
+    /**
+     * 字符包围盒缓存：offset → Rect。生命周期与本实例一致，文本或布局变化会重建实例、
+     * 缓存随之失效。仅在 UI 线程 draw 阶段访问（`drawBehind` 内按字符定位画线），单线程读写不加锁。
+     */
+    private val boundingBoxCache = HashMap<Int, Rect>()
+
     /**
      * 文本行数。提供 [lineMetricsFn] 时取自 native 回填值，否则取构造传入值
      *（如 CoreTextField 等无行度量场景）。
@@ -98,8 +104,12 @@ class MultiParagraph(
 
     /**
      * Returns the bounding box of the character for given character offset.
-     * 通过 [getBoundingBoxFn] 向 native 端查询（Android / iOS / OHOS 原生文本布局）。
+     * 通过 [getBoundingBoxFn] 向 native 端查询（Android / iOS / OHOS 原生文本布局），
+     * 同 offset 首次查询后写入 [boundingBoxCache]，之后命中缓存。
+     * [getBoundingBoxFn] 为 null（如 CoreTextField）时返回零矩形，不写缓存。
      */
-    fun getBoundingBox(offset: Int): Rect =
-        getBoundingBoxFn?.invoke(offset) ?: Rect(0f, 0f, 0f, 0f)
+    fun getBoundingBox(offset: Int): Rect {
+        val fn = getBoundingBoxFn ?: return Rect(0f, 0f, 0f, 0f)
+        return boundingBoxCache.getOrPut(offset) { fn(offset) }
+    }
 }

compose/src/commonTest/kotlin/com/tencent/kuikly/compose/ui/text/MultiParagraphTest.kt

@@ -0,0 +1,96 @@
+/*
+ * Tencent is pleased to support the open source community by making KuiklyUI
+ * available.
+ * Copyright (C) 2025 Tencent. All rights reserved.
+ * Licensed under the License of KuiklyUI;
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * https://github.com/Tencent-TDS/KuiklyUI/blob/main/LICENSE
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.tencent.kuikly.compose.ui.text
+
+import com.tencent.kuikly.compose.ui.geometry.Rect
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+/**
+ * [MultiParagraph.getBoundingBox] 的纯 Kotlin 回归用例。
+ *
+ * 核心契约：
+ * 1. 同一 [MultiParagraph] 实例里，同 offset 二次读取命中缓存，只触发一次查询。
+ * 2. 空路径（[getBoundingBoxFn] 为 null）保持返回零矩形语义，不写缓存、不抛异常。
+ */
+class MultiParagraphTest {
+
+    @Test
+    fun getBoundingBox_sameOffsetTwice_invokesFunctionOnlyOnce() {
+        var invokeCount = 0
+        val expected = Rect(1f, 2f, 3f, 4f)
+        val multiParagraph = MultiParagraph(
+            placeholderRects = emptyList(),
+            getBoundingBoxFn = { offset ->
+                invokeCount++
+                Rect(
+                    expected.left + offset,
+                    expected.top + offset,
+                    expected.right + offset,
+                    expected.bottom + offset,
+                )
+            },
+        )
+
+        val first = multiParagraph.getBoundingBox(7)
+        val second = multiParagraph.getBoundingBox(7)
+
+        assertEquals(Rect(8f, 9f, 10f, 11f), first)
+        assertEquals(first, second)
+        assertEquals(1, invokeCount)
+    }
+
+    @Test
+    fun getBoundingBox_nullFunction_returnsZeroRect() {
+        val multiParagraph = MultiParagraph(
+            initialLineCount = 2,
+            placeholderRects = emptyList(),
+        )
+
+        val first = multiParagraph.getBoundingBox(3)
+        val second = multiParagraph.getBoundingBox(3)
+
+        assertEquals(Rect(0f, 0f, 0f, 0f), first)
+        assertEquals(first, second)
+    }
+
+    @Test
+    fun getBoundingBox_differentOffsets_invokeFunctionPerOffset() {
+        var invokeCount = 0
+        val multiParagraph = MultiParagraph(
+            placeholderRects = emptyList(),
+            getBoundingBoxFn = { offset ->
+                invokeCount++
+                Rect(
+                    offset.toFloat(),
+                    offset.toFloat(),
+                    offset.toFloat() + 1f,
+                    offset.toFloat() + 1f,
+                )
+            },
+        )
+
+        val a = multiParagraph.getBoundingBox(3)
+        val b = multiParagraph.getBoundingBox(7)
+        val aAgain = multiParagraph.getBoundingBox(3)
+
+        assertEquals(Rect(3f, 3f, 4f, 4f), a)
+        assertEquals(Rect(7f, 7f, 8f, 8f), b)
+        assertEquals(a, aAgain)
+        // 两个不同 offset 各触发一次底层查询，同 offset 复读命中缓存不再触发。
+        assertEquals(2, invokeCount)
+    }
+}