diff --git a/src/react-components/Block.tsx b/src/react-components/Block.tsx
index b3a5b5ba..d775fb53 100644
--- a/src/react-components/Block.tsx
+++ b/src/react-components/Block.tsx
@@ -1,4 +1,13 @@
-import React, { ForwardedRef, forwardRef, useEffect, useImperativeHandle, useMemo, useRef, useState } from "react";
+import React, {
+  ForwardedRef,
+  forwardRef,
+  useEffect,
+  useImperativeHandle,
+  useLayoutEffect,
+  useMemo,
+  useRef,
+  useState,
+} from "react";
 
 import { TBlock } from "../components/canvas/blocks/Block";
 import { Graph } from "../graph";
@@ -6,6 +15,7 @@ import { ESchedulerPriority } from "../lib/Scheduler";
 
 import { useComputedSignal, useSchedulerDebounce, useSignalEffect } from "./hooks";
 import { useBlockState } from "./hooks/useBlockState";
+import { applyBlockContainerLayout } from "./utils/applyBlockContainerLayout";
 import { cn } from "./utils/cn";
 
 import "./Block.css";
@@ -119,45 +129,37 @@ function GraphBlockInner<T extends TBlock>(
     return () => viewState?.setHiddenBlock(false);
   }, [viewState, canvasVisible]);
 
+  /**
+   * Synchronously set initial block geometry before the browser paints
+   * to prevent blocks from flashing at position (0, 0) when mounting.
+   */
+  useLayoutEffect(() => {
+    const geometry = state?.$geometry.value;
+    const container = containerRef.current;
+    if (!container || !geometry || !viewState) {
+      return;
+    }
+    applyBlockContainerLayout(container, geometry, viewState, lastStateRef.current);
+  }, [state, viewState]);
+
   /**
    * Update the block geometry and z-index when the state changes.
    */
   useSignalEffect(() => {
     const geometry = state?.$geometry.value;
     const container = containerRef.current;
-    const lastState = lastStateRef.current;
     if (!container || !geometry || !viewState) {
       return;
     }
 
-    const hasPositionChange = lastStateRef.current.x !== geometry.x || lastStateRef.current.y !== geometry.y;
+    const { hasPositionChange } = applyBlockContainerLayout(container, geometry, viewState, lastStateRef.current);
 
     if (hasPositionChange) {
-      container.style.setProperty("--graph-block-geometry-x", `${geometry.x}px`);
-      container.style.setProperty("--graph-block-geometry-y", `${geometry.y}px`);
-      lastState.x = geometry.x;
-      lastState.y = geometry.y;
       if (!container.classList.contains("graph-block-container-moving")) {
         container.classList.add("graph-block-container-moving");
       }
       stopMoving(container);
     }
-
-    const hasSizeChange = lastState.width !== geometry.width || lastState.height !== geometry.height;
-    if (hasSizeChange) {
-      container.style.setProperty("--graph-block-geometry-width", `${geometry.width}px`);
-      container.style.setProperty("--graph-block-geometry-height", `${geometry.height}px`);
-      lastState.width = geometry.width;
-      lastState.height = geometry.height;
-    }
-
-    const { zIndex, order } = viewState.$viewState.value;
-    const newZIndex = (zIndex || 0) + (order || 0);
-
-    if (lastState.zIndex !== newZIndex) {
-      container.style.zIndex = `${newZIndex}`;
-      lastState.zIndex = newZIndex;
-    }
   }, [containerRef, lastStateRef, state, viewState]);
 
   /**
diff --git a/src/react-components/utils/applyBlockContainerLayout.ts b/src/react-components/utils/applyBlockContainerLayout.ts
new file mode 100644
index 00000000..b736bf40
--- /dev/null
+++ b/src/react-components/utils/applyBlockContainerLayout.ts
@@ -0,0 +1,43 @@
+export type BlockLayoutGeometry = { x: number; y: number; width: number; height: number };
+export type BlockLayoutLastState = { x: number; y: number; width: number; height: number; zIndex: number };
+
+/**
+ * Applies block geometry (position, size) and z-index to the container element,
+ * updating only the properties that have changed. Shared between mount-time
+ * (useLayoutEffect) and signal-driven (useSignalEffect) updates.
+ *
+ * @returns flags indicating which properties changed
+ */
+export function applyBlockContainerLayout(
+  container: HTMLDivElement,
+  geometry: BlockLayoutGeometry,
+  viewState: { $viewState: { value: { zIndex?: number; order?: number } } },
+  lastState: BlockLayoutLastState
+): { hasPositionChange: boolean; hasSizeChange: boolean } {
+  const hasPositionChange = lastState.x !== geometry.x || lastState.y !== geometry.y;
+  const hasSizeChange = lastState.width !== geometry.width || lastState.height !== geometry.height;
+
+  if (hasPositionChange) {
+    container.style.setProperty("--graph-block-geometry-x", `${geometry.x}px`);
+    container.style.setProperty("--graph-block-geometry-y", `${geometry.y}px`);
+    lastState.x = geometry.x;
+    lastState.y = geometry.y;
+  }
+
+  if (hasSizeChange) {
+    container.style.setProperty("--graph-block-geometry-width", `${geometry.width}px`);
+    container.style.setProperty("--graph-block-geometry-height", `${geometry.height}px`);
+    lastState.width = geometry.width;
+    lastState.height = geometry.height;
+  }
+
+  const { zIndex, order } = viewState.$viewState.value;
+  const newZIndex = (zIndex || 0) + (order || 0);
+
+  if (lastState.zIndex !== newZIndex) {
+    container.style.zIndex = `${newZIndex}`;
+    lastState.zIndex = newZIndex;
+  }
+
+  return { hasPositionChange, hasSizeChange };
+}