From ae5dfc0b3cf472f859643bb14f7507757b00dcb7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C3=89verton=20Toffanetto?= <evertondgn@hotmail.com>
Date: Mon, 1 Jun 2026 04:32:26 -0300
Subject: [PATCH] Fix split-pane right-edge resize clipping (#1261)

* fix(app): clamp split pane resize

Prevent right-edge drags from overflowing the Electron viewport.

* fix(app): isolate resize pointer state

Keep concurrent resize drags from clobbering each other.
---
 packages/app/public/index.html                | 16 +++-
 .../components/resize-handle-sizes.test.ts    | 67 +++++++++++++++
 .../app/src/components/resize-handle-sizes.ts | 36 ++++++++
 packages/app/src/components/resize-handle.tsx | 83 ++++++++++++++-----
 .../src/stores/workspace-layout-actions.ts    |  2 +-
 .../src/stores/workspace-layout-constants.ts  |  1 +
 6 files changed, 181 insertions(+), 24 deletions(-)
 create mode 100644 packages/app/src/components/resize-handle-sizes.test.ts
 create mode 100644 packages/app/src/components/resize-handle-sizes.ts
 create mode 100644 packages/app/src/stores/workspace-layout-constants.ts

diff --git a/packages/app/public/index.html b/packages/app/public/index.html
index 6e926b512..f7116fc86 100644
--- a/packages/app/public/index.html
+++ b/packages/app/public/index.html
@@ -17,14 +17,18 @@
     <title>%WEB_TITLE%</title>
     <!-- The `react-native-web` recommended style reset: https://necolas.github.io/react-native-web/docs/setup/#root-element -->
     <style id="expo-reset">
-      /* These styles make the body full-height */
+      /* Keep the app shell fixed to the viewport. */
       html,
       body {
+        width: 100%;
         height: 100%;
-      }
-      /* These styles disable body scrolling if you are using <ScrollView> */
-      body {
+        margin: 0;
+        padding: 0;
         overflow: hidden;
+        overscroll-behavior: none;
+      }
+      /* These styles smooth text rendering in the app shell. */
+      body {
         -webkit-font-smoothing: antialiased;
         -moz-osx-font-smoothing: grayscale;
       }
@@ -38,8 +42,12 @@
       /* These styles make the root element full-height */
       #root {
         display: flex;
+        width: 100%;
         height: 100%;
         flex: 1;
+        min-width: 0;
+        min-height: 0;
+        overflow: hidden;
       }
     </style>
     <style>
diff --git a/packages/app/src/components/resize-handle-sizes.test.ts b/packages/app/src/components/resize-handle-sizes.test.ts
new file mode 100644
index 000000000..ecfe398dd
--- /dev/null
+++ b/packages/app/src/components/resize-handle-sizes.test.ts
@@ -0,0 +1,67 @@
+import { describe, expect, it } from "vitest";
+import { computeResizeHandleSizes } from "@/components/resize-handle-sizes";
+
+describe("computeResizeHandleSizes", () => {
+  it("clamps right-edge drags to the adjacent pane minimum", () => {
+    const sizes = computeResizeHandleSizes({
+      sizes: [0.25, 0.5, 0.25],
+      index: 1,
+      deltaRatio: 0.5,
+    });
+
+    expect(sizes[0]).toBe(0.25);
+    expect(sizes[1]).toBe(0.65);
+    expect(sizes[2]).toBeCloseTo(0.1, 10);
+  });
+
+  it("clamps left-edge drags to the adjacent pane minimum", () => {
+    const sizes = computeResizeHandleSizes({
+      sizes: [0.25, 0.5, 0.25],
+      index: 1,
+      deltaRatio: -0.5,
+    });
+
+    expect(sizes[0]).toBe(0.25);
+    expect(sizes[1]).toBe(0.1);
+    expect(sizes[2]).toBeCloseTo(0.65, 10);
+  });
+
+  it("moves adjacent pane sizes without clamping", () => {
+    const sizes = computeResizeHandleSizes({
+      sizes: [0.25, 0.5, 0.25],
+      index: 1,
+      deltaRatio: 0.05,
+    });
+
+    expect(sizes[0]).toBe(0.25);
+    expect(sizes[1]).toBe(0.55);
+    expect(sizes[2]).toBeCloseTo(0.2, 10);
+  });
+
+  it("splits tiny adjacent pairs evenly when the configured minimum cannot fit", () => {
+    expect(
+      computeResizeHandleSizes({
+        sizes: [0.45, 0.05, 0.05, 0.45],
+        index: 1,
+        deltaRatio: 0.05,
+      }),
+    ).toEqual([0.45, 0.05, 0.05, 0.45]);
+  });
+
+  it("leaves sizes unchanged when the adjacent pair is invalid", () => {
+    expect(
+      computeResizeHandleSizes({
+        sizes: [0.25, 0.5, 0.25],
+        index: 3,
+        deltaRatio: 0.25,
+      }),
+    ).toEqual([0.25, 0.5, 0.25]);
+    expect(
+      computeResizeHandleSizes({
+        sizes: [0.25, 0, 0, 0.75],
+        index: 1,
+        deltaRatio: 0.25,
+      }),
+    ).toEqual([0.25, 0, 0, 0.75]);
+  });
+});
diff --git a/packages/app/src/components/resize-handle-sizes.ts b/packages/app/src/components/resize-handle-sizes.ts
new file mode 100644
index 000000000..9d4e13f32
--- /dev/null
+++ b/packages/app/src/components/resize-handle-sizes.ts
@@ -0,0 +1,36 @@
+import { MIN_SPLIT_SIZE } from "@/stores/workspace-layout-constants";
+
+interface ComputeResizeHandleSizesInput {
+  sizes: number[];
+  index: number;
+  deltaRatio: number;
+  minSize?: number;
+}
+
+export function computeResizeHandleSizes({
+  sizes,
+  index,
+  deltaRatio,
+  minSize = MIN_SPLIT_SIZE,
+}: ComputeResizeHandleSizesInput): number[] {
+  const nextSizes = sizes.slice();
+  const leftSize = sizes[index];
+  const rightSize = sizes[index + 1];
+  if (leftSize === undefined || rightSize === undefined) {
+    return nextSizes;
+  }
+
+  const pairSize = leftSize + rightSize;
+  if (pairSize <= 0) {
+    return nextSizes;
+  }
+
+  const adjacentMinSize = Math.min(minSize, pairSize / 2);
+  const nextLeftSize = Math.min(
+    pairSize - adjacentMinSize,
+    Math.max(adjacentMinSize, leftSize + deltaRatio),
+  );
+  nextSizes[index] = nextLeftSize;
+  nextSizes[index + 1] = pairSize - nextLeftSize;
+  return nextSizes;
+}
diff --git a/packages/app/src/components/resize-handle.tsx b/packages/app/src/components/resize-handle.tsx
index 651e4380a..06b020cd5 100644
--- a/packages/app/src/components/resize-handle.tsx
+++ b/packages/app/src/components/resize-handle.tsx
@@ -1,6 +1,7 @@
 import { useCallback, useMemo, useRef, useState } from "react";
 import { View, type PointerEvent as RNPointerEvent } from "react-native";
 import { StyleSheet, useUnistyles } from "react-native-unistyles";
+import { computeResizeHandleSizes } from "@/components/resize-handle-sizes";
 
 export interface ResizeHandleProps {
   direction: "horizontal" | "vertical";
@@ -13,8 +14,14 @@ export interface ResizeHandleProps {
 interface PointerState {
   containerSize: number;
   pointerStart: number;
-  leftSize: number;
-  rightSize: number;
+}
+
+function resetWindowHorizontalScroll() {
+  // Clamp any browser scroll introduced while dragging past the viewport edge.
+  if (window.scrollX === 0) {
+    return;
+  }
+  window.scrollTo(0, window.scrollY);
 }
 
 export function ResizeHandle({
@@ -25,7 +32,8 @@ export function ResizeHandle({
   onResizeSplit,
 }: ResizeHandleProps) {
   const { theme } = useUnistyles();
-  const pointerStateRef = useRef<PointerState | null>(null);
+  const pointerStatesRef = useRef(new Map<number, PointerState>());
+  const cursorBeforeDragRef = useRef<string | null>(null);
   const hoverTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
   const [active, setActive] = useState(false);
   const [dragging, setDragging] = useState(false);
@@ -34,7 +42,11 @@ export function ResizeHandle({
   const handlePointerDown = useCallback(
     (event: RNPointerEvent) => {
       const hitAreaElement = event.currentTarget as unknown as HTMLElement | null;
-      const containerElement = hitAreaElement?.parentElement?.parentElement ?? null;
+      if (!hitAreaElement) {
+        return;
+      }
+
+      const containerElement = hitAreaElement.parentElement?.parentElement ?? null;
       if (!containerElement) {
         return;
       }
@@ -45,51 +57,83 @@ export function ResizeHandle({
         return;
       }
 
+      const pointerId = event.nativeEvent.pointerId;
+      if (pointerStatesRef.current.has(pointerId)) {
+        return;
+      }
+
       setDragging(true);
 
-      pointerStateRef.current = {
+      pointerStatesRef.current.set(pointerId, {
         containerSize,
         pointerStart:
           direction === "horizontal" ? event.nativeEvent.clientX : event.nativeEvent.clientY,
-        leftSize: sizes[index] ?? 0,
-        rightSize: sizes[index + 1] ?? 0,
-      };
+      });
 
-      const previousCursor = document.body.style.cursor;
+      if (pointerStatesRef.current.size === 1) {
+        cursorBeforeDragRef.current = document.body.style.cursor;
+      }
       const nextCursor = direction === "horizontal" ? "col-resize" : "row-resize";
       document.body.style.cursor = nextCursor;
       event.preventDefault();
+      event.stopPropagation();
+      const pointerCaptureElement = hitAreaElement;
+      pointerCaptureElement.setPointerCapture?.(pointerId);
+      resetWindowHorizontalScroll();
 
       function cleanup() {
-        pointerStateRef.current = null;
-        setDragging(false);
-        document.body.style.cursor = previousCursor;
+        pointerStatesRef.current.delete(pointerId);
+        setDragging(pointerStatesRef.current.size > 0);
+        if (pointerStatesRef.current.size === 0) {
+          document.body.style.cursor = cursorBeforeDragRef.current ?? "";
+          cursorBeforeDragRef.current = null;
+        }
+        if (pointerCaptureElement.hasPointerCapture?.(pointerId)) {
+          pointerCaptureElement.releasePointerCapture(pointerId);
+        }
+        resetWindowHorizontalScroll();
         window.removeEventListener("pointermove", handlePointerMove);
         window.removeEventListener("pointerup", handlePointerUp);
+        window.removeEventListener("pointercancel", handlePointerUp);
       }
 
       function handlePointerMove(moveEvent: PointerEvent) {
-        const pointerState = pointerStateRef.current;
+        if (moveEvent.pointerId !== pointerId) {
+          return;
+        }
+
+        const pointerState = pointerStatesRef.current.get(pointerId);
         if (!pointerState) {
           return;
         }
 
+        moveEvent.preventDefault();
+        resetWindowHorizontalScroll();
         const pointerCurrent = direction === "horizontal" ? moveEvent.clientX : moveEvent.clientY;
         const deltaRatio =
           (pointerCurrent - pointerState.pointerStart) / pointerState.containerSize;
 
-        const nextSizes = sizes.slice();
-        nextSizes[index] = pointerState.leftSize + deltaRatio;
-        nextSizes[index + 1] = pointerState.rightSize - deltaRatio;
-        onResizeSplit(groupId, nextSizes);
+        onResizeSplit(
+          groupId,
+          computeResizeHandleSizes({
+            sizes,
+            index,
+            deltaRatio,
+          }),
+        );
       }
 
-      function handlePointerUp() {
+      function handlePointerUp(upEvent: PointerEvent) {
+        if (upEvent.pointerId !== pointerId) {
+          return;
+        }
+
         cleanup();
       }
 
       window.addEventListener("pointermove", handlePointerMove);
-      window.addEventListener("pointerup", handlePointerUp, { once: true });
+      window.addEventListener("pointerup", handlePointerUp);
+      window.addEventListener("pointercancel", handlePointerUp);
     },
     [direction, groupId, index, onResizeSplit, sizes],
   );
@@ -130,6 +174,7 @@ export function ResizeHandle({
       direction === "horizontal" ? styles.hitAreaHorizontal : styles.hitAreaVertical,
       {
         cursor: direction === "horizontal" ? "col-resize" : "row-resize",
+        touchAction: "none",
       } as object,
     ],
     [direction],
diff --git a/packages/app/src/stores/workspace-layout-actions.ts b/packages/app/src/stores/workspace-layout-actions.ts
index cd6fb0532..86cba617c 100644
--- a/packages/app/src/stores/workspace-layout-actions.ts
+++ b/packages/app/src/stores/workspace-layout-actions.ts
@@ -1,5 +1,6 @@
 import invariant from "tiny-invariant";
 import type { WorkspaceTab, WorkspaceTabTarget } from "@/stores/workspace-tabs-store";
+import { MIN_SPLIT_SIZE } from "@/stores/workspace-layout-constants";
 import { defaultWorkspaceLayoutIds } from "@/stores/workspace-layout-ids";
 import type { WorkspaceLayoutNodeIdPrefix } from "@/stores/workspace-layout-ids";
 import {
@@ -208,7 +209,6 @@ export interface WorkspaceTabSnapshot {
 }
 
 const DEFAULT_PANE_ID = "main";
-const MIN_SPLIT_SIZE = 0.1;
 
 function trimNonEmpty(value: string | null | undefined): string | null {
   if (typeof value !== "string") {
diff --git a/packages/app/src/stores/workspace-layout-constants.ts b/packages/app/src/stores/workspace-layout-constants.ts
new file mode 100644
index 000000000..b0cfb3826
--- /dev/null
+++ b/packages/app/src/stores/workspace-layout-constants.ts
@@ -0,0 +1 @@
+export const MIN_SPLIT_SIZE = 0.1;