UI Components Hard

Resizable Panels

Draggable split panels — horizontal and vertical resize, with min/max constraints, collapse/expand, and a multi-panel layout.

Open in Lab

css vanilla-js

Targets: JS HTML

Code

*,
*::before,
*::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

:root {
  --bg: #050910;
  --card: #0d1117;
  --card2: #0a0f1a;
  --border: rgba(255, 255, 255, 0.08);
  --text: #f2f6ff;
  --muted: #475569;
  --accent: #38bdf8;
  --success: #4ade80;
  --radius: 12px;
}

body {
  background: var(--bg);
  color: var(--text);
  font-family: system-ui, -apple-system, sans-serif;
  min-height: 100vh;
  padding: 2.5rem 1.5rem;
}

.page {
  max-width: 900px;
  margin: 0 auto;
  display: flex;
  flex-direction: column;
  gap: 2.5rem;
}

.demo-section {
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
}

.demo-label {
  font-size: 0.75rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.09em;
  color: var(--muted);
}

/* ── Panels container ── */
.panels {
  border: 1px solid var(--border);
  border-radius: var(--radius);
  overflow: hidden;
  display: flex;
  height: 260px;
  background: var(--card);
  user-select: none;
}

.panels--v {
  flex-direction: column;
}

/* ── Panel ── */
.panel {
  flex: 1;
  min-width: 80px;
  min-height: 60px;
  overflow: hidden;
  display: flex;
  flex-direction: column;
  flex-shrink: 0;
}

.panel-sidebar {
  flex-shrink: 0;
}

/* ── Panel header ── */
.panel-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0.5rem 0.875rem;
  border-bottom: 1px solid var(--border);
  background: var(--card2);
  flex-shrink: 0;
  min-height: 36px;
}

.panel-title {
  font-size: 0.75rem;
  font-weight: 600;
  color: var(--muted);
  letter-spacing: 0.04em;
}

.panel-tabs {
  display: flex;
  gap: 0.5rem;
}

.panel-tab {
  font-size: 0.72rem;
  color: var(--muted);
  padding: 0.2rem 0.5rem;
  border-radius: 4px;
  cursor: pointer;
}

.panel-tab.active {
  background: rgba(56, 189, 248, 0.1);
  color: var(--accent);
}

/* ── Panel body ── */
.panel-body {
  flex: 1;
  overflow: auto;
  padding: 0.625rem;
  scrollbar-width: thin;
  scrollbar-color: rgba(255, 255, 255, 0.08) transparent;
}

.panel-body::-webkit-scrollbar {
  width: 4px;
}
.panel-body::-webkit-scrollbar-track {
  background: transparent;
}
.panel-body::-webkit-scrollbar-thumb {
  background: rgba(255, 255, 255, 0.1);
  border-radius: 2px;
}

/* ── Resize handle ── */
.resize-handle {
  flex-shrink: 0;
  background: var(--border);
  position: relative;
  z-index: 1;
  transition: background 0.15s;
}

.resize-handle--h {
  width: 4px;
  cursor: col-resize;
}

.resize-handle--v {
  height: 4px;
  cursor: row-resize;
}

.resize-handle:hover,
.resize-handle:focus-visible,
.resize-handle.dragging {
  background: var(--accent);
  outline: none;
}

/* Widen hit area without changing visual width */
.resize-handle--h::before {
  content: "";
  position: absolute;
  top: 0;
  left: -4px;
  right: -4px;
  bottom: 0;
}

.resize-handle--v::before {
  content: "";
  position: absolute;
  left: 0;
  top: -4px;
  right: 0;
  bottom: -4px;
}

/* ── File tree ── */
.file-tree {
  list-style: none;
  font-size: 0.8rem;
  line-height: 2.2;
}

.ft-dir > span,
.ft-file {
  display: flex;
  align-items: center;
  gap: 0.3rem;
  padding: 0 0.25rem;
  border-radius: 4px;
  cursor: pointer;
  white-space: nowrap;
  color: var(--muted);
  transition: background 0.15s, color 0.15s;
}

.ft-dir > span:hover,
.ft-file:hover {
  background: rgba(255, 255, 255, 0.04);
  color: var(--text);
}

.ft-file.active {
  background: rgba(56, 189, 248, 0.08);
  color: var(--accent);
}

.ft-dir ul {
  list-style: none;
  padding-left: 1rem;
}

/* ── Code block ── */
.panel-body--code {
  padding: 0.875rem;
}

.code-block {
  font-family: "SF Mono", "Fira Code", monospace;
  font-size: 0.78rem;
  line-height: 1.8;
  color: var(--text);
  white-space: pre;
}

.c-tag {
  color: #7dd3fc;
}
.c-attr {
  color: #86efac;
}
.c-str {
  color: #fde68a;
}
.c-cmt {
  color: var(--muted);
  font-style: italic;
}

/* ── Preview ── */
.panel-body--preview {
  display: flex;
  align-items: center;
  justify-content: center;
}

.preview-box {
  width: 80%;
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.preview-bar {
  height: 10px;
  background: rgba(56, 189, 248, 0.15);
  border-radius: 5px;
}

.preview-bar--short {
  width: 45%;
  background: rgba(56, 189, 248, 0.08);
}
.preview-bar--medium {
  width: 70%;
  background: rgba(56, 189, 248, 0.08);
}

/* ── Terminal ── */
.terminal-dots {
  display: flex;
  gap: 5px;
}

.tdot {
  width: 10px;
  height: 10px;
  border-radius: 50%;
}

.tdot--red {
  background: #f87171;
}
.tdot--yellow {
  background: #fbbf24;
}
.tdot--green {
  background: #4ade80;
}

.panel-body--terminal {
  padding: 0.625rem 0.875rem;
  font-family: "SF Mono", "Fira Code", monospace;
  font-size: 0.78rem;
}

.term-line {
  line-height: 2;
  color: var(--text);
}

.term-prompt {
  color: var(--accent);
  margin-right: 0.5rem;
}

.term-line--ok {
  color: #4ade80;
}
.term-line--dim {
  color: var(--muted);
}

/* ── Three-panel ── */
.placeholder-list {
  display: flex;
  flex-direction: column;
}

.pl-item {
  padding: 0.4rem 0.5rem;
  font-size: 0.82rem;
  color: var(--muted);
  border-radius: 5px;
  cursor: pointer;
  transition: background 0.15s, color 0.15s;
}

.pl-item:hover {
  background: rgba(255, 255, 255, 0.04);
  color: var(--text);
}
.pl-item.active {
  background: rgba(56, 189, 248, 0.08);
  color: var(--accent);
}

.panel-body--center-placeholder {
  display: flex;
  align-items: center;
  justify-content: center;
}

.center-msg {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.5rem;
  color: var(--muted);
  font-size: 0.85rem;
}

.prop-row {
  display: flex;
  justify-content: space-between;
  padding: 0.35rem 0.25rem;
  font-size: 0.8rem;
  border-bottom: 1px solid rgba(255, 255, 255, 0.04);
}

.prop-key {
  color: var(--muted);
}
.prop-val {
  color: var(--accent);
  font-variant-numeric: tabular-nums;
}

(function () {
  "use strict";

  const MIN = 80; // px minimum per panel

  // ── Init all resize handles ──────────────────────────────────────────────────

  document.querySelectorAll(".panels").forEach(function (container) {
    const dir = container.dataset.direction || "horizontal";
    const handles = Array.from(container.querySelectorAll(".resize-handle"));

    handles.forEach(function (handle) {
      initHandle(handle, container, dir);
    });
  });

  // ── Per-handle setup ─────────────────────────────────────────────────────────

  function initHandle(handle, container, dir) {
    const isH = dir === "horizontal";

    // Find the panels on each side of this handle
    function getPanels() {
      const children = Array.from(container.children).filter(function (c) {
        return !c.classList.contains("resize-handle");
      });
      const handles = Array.from(container.querySelectorAll(".resize-handle"));
      const hIdx = handles.indexOf(handle);
      return { before: children[hIdx], after: children[hIdx + 1] };
    }

    let startPos = 0;
    let startBefore = 0;
    let startAfter = 0;
    let collapsed = false;
    let savedBefore = 0;
    let dragging = false;

    // ── Drag ──────────────────────────────────────────────────────────────────

    function onDown(e) {
      e.preventDefault();
      dragging = true;
      handle.classList.add("dragging");

      const { before, after } = getPanels();
      startPos = isH ? getClientX(e) : getClientY(e);
      startBefore = isH ? before.offsetWidth : before.offsetHeight;
      startAfter = isH ? after.offsetWidth : after.offsetHeight;

      document.addEventListener("mousemove", onMove);
      document.addEventListener("touchmove", onMove, { passive: false });
      document.addEventListener("mouseup", onUp);
      document.addEventListener("touchend", onUp);
    }

    function onMove(e) {
      if (!dragging) return;
      e.preventDefault();

      const { before, after } = getPanels();
      const pos = isH ? getClientX(e) : getClientY(e);
      const delta = pos - startPos;
      const total = startBefore + startAfter;

      let newBefore = Math.max(MIN, Math.min(total - MIN, startBefore + delta));
      let newAfter = total - newBefore;

      if (isH) {
        before.style.flexBasis = newBefore + "px";
        before.style.flex = "0 0 " + newBefore + "px";
        after.style.flex = "1";
      } else {
        before.style.flexBasis = newBefore + "px";
        before.style.flex = "0 0 " + newBefore + "px";
        after.style.flex = "1";
      }

      // Update ARIA
      handle.setAttribute("aria-valuenow", Math.round(newBefore));
    }

    function onUp() {
      dragging = false;
      handle.classList.remove("dragging");
      document.removeEventListener("mousemove", onMove);
      document.removeEventListener("touchmove", onMove);
      document.removeEventListener("mouseup", onUp);
      document.removeEventListener("touchend", onUp);
    }

    handle.addEventListener("mousedown", onDown);
    handle.addEventListener("touchstart", onDown, { passive: false });

    // ── Double-click to collapse/restore ──────────────────────────────────────

    handle.addEventListener("dblclick", function () {
      const { before, after } = getPanels();
      if (collapsed) {
        // Restore
        before.style.flex = "0 0 " + savedBefore + "px";
        after.style.flex = "1";
        collapsed = false;
      } else {
        // Collapse
        savedBefore = isH ? before.offsetWidth : before.offsetHeight;
        before.style.flex = "0 0 0px";
        after.style.flex = "1";
        collapsed = true;
      }
    });

    // ── Keyboard resize ───────────────────────────────────────────────────────

    handle.addEventListener("keydown", function (e) {
      const { before, after } = getPanels();
      const step = 20;
      const current = isH ? before.offsetWidth : before.offsetHeight;
      let next = current;

      if (isH) {
        if (e.key === "ArrowLeft") next = Math.max(MIN, current - step);
        if (e.key === "ArrowRight")
          next = Math.min(before.offsetWidth + after.offsetWidth - MIN, current + step);
      } else {
        if (e.key === "ArrowUp") next = Math.max(MIN, current - step);
        if (e.key === "ArrowDown")
          next = Math.min(before.offsetHeight + after.offsetHeight - MIN, current + step);
      }

      if (next !== current) {
        e.preventDefault();
        before.style.flex = "0 0 " + next + "px";
        after.style.flex = "1";
        handle.setAttribute("aria-valuenow", next);
      }
    });
  }

  // ── Pointer helpers ──────────────────────────────────────────────────────────

  function getClientX(e) {
    return e.touches ? e.touches[0].clientX : e.clientX;
  }
  function getClientY(e) {
    return e.touches ? e.touches[0].clientY : e.clientY;
  }
})();

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Resizable Panels</title>
  <link rel="stylesheet" href="style.css" />
</head>
<body>

  <div class="page">

    <!-- ── Demo 1: Horizontal two-panel ── -->
    <div class="demo-section">
      <h2 class="demo-label">Horizontal — left sidebar | main content</h2>
      <div class="panels panels--h" id="panels-h" data-direction="horizontal">

        <div class="panel panel-sidebar" id="panel-h-left" style="flex-basis: 220px;">
          <div class="panel-header">
            <span class="panel-title">File Tree</span>
          </div>
          <div class="panel-body">
            <ul class="file-tree">
              <li class="ft-dir open">
                <span>📁 src</span>
                <ul>
                  <li class="ft-file active">📄 index.html</li>
                  <li class="ft-file">📄 style.css</li>
                  <li class="ft-file">📄 script.js</li>
                </ul>
              </li>
              <li class="ft-dir">
                <span>📁 components</span>
              </li>
              <li class="ft-file">📄 package.json</li>
              <li class="ft-file">📄 README.md</li>
            </ul>
          </div>
        </div>

        <div class="resize-handle resize-handle--h" id="rh-h-1" tabindex="0" role="separator" aria-orientation="vertical" aria-label="Resize sidebar" aria-valuenow="220" aria-valuemin="80" aria-valuemax="500" title="Drag to resize · Double-click to collapse"></div>

        <div class="panel" id="panel-h-right">
          <div class="panel-header">
            <span class="panel-title">Editor — index.html</span>
            <div class="panel-tabs">
              <span class="panel-tab active">index.html</span>
              <span class="panel-tab">style.css</span>
            </div>
          </div>
          <div class="panel-body panel-body--code">
            <pre class="code-block"><code><span class="c-tag">&lt;!DOCTYPE html&gt;</span>
<span class="c-tag">&lt;html</span> <span class="c-attr">lang</span>=<span class="c-str">"en"</span><span class="c-tag">&gt;</span>
<span class="c-tag">&lt;head&gt;</span>
  <span class="c-tag">&lt;meta</span> <span class="c-attr">charset</span>=<span class="c-str">"UTF-8"</span> <span class="c-tag">/&gt;</span>
  <span class="c-tag">&lt;title&gt;</span>My Page<span class="c-tag">&lt;/title&gt;</span>
<span class="c-tag">&lt;/head&gt;</span>
<span class="c-tag">&lt;body&gt;</span>
  <span class="c-cmt">&lt;!-- content --&gt;</span>
<span class="c-tag">&lt;/body&gt;</span>
<span class="c-tag">&lt;/html&gt;</span></code></pre>
          </div>
        </div>
      </div>
    </div>

    <!-- ── Demo 2: Vertical two-panel ── -->
    <div class="demo-section">
      <h2 class="demo-label">Vertical — top | bottom</h2>
      <div class="panels panels--v" id="panels-v" data-direction="vertical" style="height: 280px;">

        <div class="panel" id="panel-v-top" style="flex-basis: 130px;">
          <div class="panel-header">
            <span class="panel-title">Preview</span>
          </div>
          <div class="panel-body panel-body--preview">
            <div class="preview-box">
              <div class="preview-bar"></div>
              <div class="preview-bar preview-bar--short"></div>
              <div class="preview-bar preview-bar--medium"></div>
            </div>
          </div>
        </div>

        <div class="resize-handle resize-handle--v" id="rh-v-1" tabindex="0" role="separator" aria-orientation="horizontal" aria-label="Resize panels" aria-valuenow="130" aria-valuemin="60" aria-valuemax="220" title="Drag to resize · Double-click to collapse"></div>

        <div class="panel" id="panel-v-bottom">
          <div class="panel-header">
            <span class="panel-title">Terminal</span>
            <div class="terminal-dots">
              <span class="tdot tdot--red"></span>
              <span class="tdot tdot--yellow"></span>
              <span class="tdot tdot--green"></span>
            </div>
          </div>
          <div class="panel-body panel-body--terminal">
            <div class="term-line"><span class="term-prompt">$</span> bun run dev</div>
            <div class="term-line term-line--ok">✓ Server listening on http://localhost:4321</div>
            <div class="term-line term-line--dim">watching for changes…</div>
          </div>
        </div>

      </div>
    </div>

    <!-- ── Demo 3: Three-panel ── -->
    <div class="demo-section">
      <h2 class="demo-label">Three-panel — left | center | right</h2>
      <div class="panels panels--h" id="panels-3" data-direction="horizontal">

        <div class="panel" id="panel-3-left" style="flex-basis: 180px;">
          <div class="panel-header">
            <span class="panel-title">Explorer</span>
          </div>
          <div class="panel-body">
            <div class="placeholder-list">
              <div class="pl-item active">Components</div>
              <div class="pl-item">Pages</div>
              <div class="pl-item">Layouts</div>
              <div class="pl-item">Styles</div>
            </div>
          </div>
        </div>

        <div class="resize-handle resize-handle--h" tabindex="0" role="separator" aria-orientation="vertical" aria-label="Resize left panel" title="Drag to resize · Double-click to collapse"></div>

        <div class="panel" id="panel-3-center">
          <div class="panel-header">
            <span class="panel-title">Editor</span>
          </div>
          <div class="panel-body panel-body--center-placeholder">
            <div class="center-msg">
              <span>📝</span>
              <span>Select a file to edit</span>
            </div>
          </div>
        </div>

        <div class="resize-handle resize-handle--h" tabindex="0" role="separator" aria-orientation="vertical" aria-label="Resize right panel" title="Drag to resize · Double-click to collapse"></div>

        <div class="panel" id="panel-3-right" style="flex-basis: 200px;">
          <div class="panel-header">
            <span class="panel-title">Properties</span>
          </div>
          <div class="panel-body">
            <div class="prop-row">
              <span class="prop-key">Width</span>
              <span class="prop-val">auto</span>
            </div>
            <div class="prop-row">
              <span class="prop-key">Height</span>
              <span class="prop-val">100%</span>
            </div>
            <div class="prop-row">
              <span class="prop-key">Display</span>
              <span class="prop-val">flex</span>
            </div>
            <div class="prop-row">
              <span class="prop-key">Gap</span>
              <span class="prop-val">0px</span>
            </div>
          </div>
        </div>

      </div>
    </div>

  </div>

  <script src="script.js"></script>
</body>
</html>

Resizable Panels

Draggable split-panel layouts with horizontal and vertical orientations, min/max size constraints, and double-click collapse. Three demos show common IDE-style and dashboard layouts.

Variants

Horizontal two-panel — left sidebar | main content
Vertical two-panel — top pane | bottom pane
Three-panel — left | center | right

How it works

Panels use display: flex with flex-basis as the sizing unit
mousedown / touchstart on a resize handle begins tracking pointer position
mousemove / touchmove updates the adjacent panels’ flex-basis values in real time
A minimum size of 80px is enforced on each panel; the max is the container width minus the opposing minimum
mouseup / touchend ends the drag and fires a resize event on the container

Collapse / expand

Double-clicking a handle collapses the left (or top) panel to 0px and saves the previous size. A second double-click restores it. Collapsed panels show a thin strip that remains draggable.