diff --git a/js/ui/menu-size-adjust.js b/js/ui/menu-size-adjust.js
new file mode 100644
index 0000000..84caa63
--- /dev/null
+++ b/js/ui/menu-size-adjust.js
@@ -0,0 +1,118 @@
+export default class MenuResizer {
+  constructor(menu) {
+    const menuHandleClass = 'menu-size-adjust-handle';
+    const menuElm = menu.getRootPane();
+    const menuHandle = document.createElement('div');
+    menuHandle.classList.add(menuHandleClass);
+    menuElm.appendChild(menuHandle);
+
+    const timelineHandleClass = 'timeline-size-adjust-handle';
+    const timelineElm = menu.getTimeline().getRootPane();
+    const timelineHandle = document.createElement('div');
+    timelineHandle.classList.add(timelineHandleClass);
+    timelineElm.appendChild(timelineHandle);
+
+    const styleElm = document.createElement('style');
+    document.body.appendChild(styleElm);
+    const stylesheet = styleElm.sheet;
+    // don't enable this feature on mobile
+    if (menu.isCoverFullWidth()) {
+      this.stylesheet.disabled = true;
+    }
+    stylesheet.insertRule(`
+      .${menuHandleClass} {
+        display: block;
+      }
+    `);
+    stylesheet.insertRule(`
+      .${timelineHandleClass} {
+        display: block;
+      }
+    `);
+    window.addEventListener('resize', () => this.onresize());
+
+    this.menu = menu;
+    this.stylesheet = stylesheet;
+    this.menuElm = menuElm;
+    this.menuHandle = menuHandle;
+    this.timelineElm = timelineElm;
+    this.timelineHandle = timelineHandle;
+
+    this.setupMenuResizing();
+    this.setupTimelineResizing();
+  }
+
+  onresize() {
+    // TODO update dimensions to match new window size
+  }
+
+  setupMenuResizing() {
+    const cssRules = this.stylesheet.cssRules;
+    // FIXME this whole approach of continuously adding and removing
+    // rules is prone to index-invalidation issues.
+    // However, since the toggle button has two identities:
+    // 1. open menu  *outside* menu itself
+    // 2. close menu being (seemingly) part of it
+    // setting the style directly is impossible since we need the
+    // :checked selector
+    let toggleBtnLeft = 0;
+    const toggleBtnSelector = '.menu-container > input:checked + label[title="Toggle menu"]';
+    let toggleBtnRule = this.stylesheet.insertRule(`
+      ${toggleBtnSelector} {
+        /* No rule needed yet */
+      }
+    `, cssRules.length);
+
+    this.menuHandle.addEventListener('mousedown', (evt) => {
+      let prevX = evt.clientX;
+      let menuWidth = this.menuElm.offsetWidth;
+      let timelineWidth = this.timelineElm.offsetWidth;
+      const onDrag = (evt) => {
+        const delta = evt.clientX - prevX;
+        prevX = evt.clientX;
+
+        menuWidth += delta;
+        this.menuElm.setAttribute('style', `width: ${menuWidth}px`);
+
+        timelineWidth -= delta;
+        this.timelineElm.style.width = `${timelineWidth}px`;
+        this.timelineElm.style.right = '0';
+        this.timelineElm.style.left = 'unset';
+
+        toggleBtnLeft += delta;
+        this.stylesheet.removeRule(toggleBtnRule);
+        toggleBtnRule = this.stylesheet.insertRule(`
+          ${toggleBtnSelector} {
+            transform: translateX(-100%) translateX(${toggleBtnLeft}px) !important;
+          }
+        `, cssRules.length);
+      };
+      const onDragEnd = (evt) => {
+        document.documentElement.removeEventListener('mousemove', onDrag);
+        document.documentElement.removeEventListener('mouseup', onDragEnd);
+      };
+      document.documentElement.addEventListener('mousemove', onDrag);
+      document.documentElement.addEventListener('mouseup', onDragEnd);
+    });
+  }
+
+  setupTimelineResizing() {
+    this.timelineHandle.addEventListener('mousedown', (evt) => {
+      let prevY = evt.clientY;
+      let timelineHeight = this.timelineElm.offsetHeight;
+      const onDrag = (evt) => {
+        const delta = evt.clientY - prevY;
+        prevY = evt.clientY;
+
+        timelineHeight -= delta;
+        this.timelineElm.style.height = `${timelineHeight}px`;
+      };
+      const onDragEnd = (evt) => {
+        document.documentElement.removeEventListener('mousemove', onDrag);
+        document.documentElement.removeEventListener('mouseup', onDragEnd);
+      };
+      document.documentElement.addEventListener('mousemove', onDrag);
+      document.documentElement.addEventListener('mouseup', onDragEnd);
+    });
+  }
+}
diff --git a/js/ui/menu.js b/js/ui/menu.js
index a9852a1..070bb10 100644
--- a/js/ui/menu.js
+++ b/js/ui/menu.js
@@ -3,6 +3,7 @@ import Config from '../config';
 import Timeline from './timeline';
 import Control from './control';
 import PresetSelect from './menu-preset-select';
+import MenuResizer from './menu-size-adjust';
 import { parseHtml } from './util';
 
 import EffectConfig from '../effects/effect-config';
@@ -437,6 +438,8 @@ export default class MainMenu {
     }
     this.timeline.loadTimeline(tracks);
 
+    this.menuResizer = new MenuResizer(this);
+
     this.submittedConfig = this.readConfig();
   }
 
@@ -483,6 +486,16 @@ export default class MainMenu {
     return parseInt(menuWidth, 10) === window.innerWidth;
   }
 
+  /// @return the dom element serving as the top-level container of all
+  ///         menu elements and also defining the menu's bounding box
+  getRootPane() {
+    return this.menuContent;
+  }
+
+  getTimeline() {
+    return this.timeline;
+  }
+
   notifyChange() {
     this.applyBtn.disabled = false;
   }
diff --git a/js/ui/timeline.js b/js/ui/timeline.js
index da9731b..f8dc061 100644
--- a/js/ui/timeline.js
+++ b/js/ui/timeline.js
@@ -763,4 +763,9 @@ export default class Timeline {
       }
     }
   }
+  /// @return the dom element serving as the top-level container of all
+  ///         timeline elements and also defining the timeline's bounding box
+  getRootPane() {
+    return this.element;
+  }
 }
diff --git a/sass/_menu-size-adjust.scss b/sass/_menu-size-adjust.scss
new file mode 100644
index 0000000..01d8b16
--- /dev/null
+++ b/sass/_menu-size-adjust.scss
@@ -0,0 +1,25 @@
+@import 'mixins/unselectable';
+
+.menu-size-adjust-handle {
+  position: absolute;
+  top: 0;
+  right: 0;
+  width: 3px;
+  height: 100%;
+  cursor: ew-resize;
+  @include unselectable;
+
+  display: none; /* this will be overridden by js */
+}
+
+.timeline-size-adjust-handle {
+  position: absolute;
+  top: 0;
+  left: 0;
+  width: 100%;
+  height: 3px;
+  cursor: ns-resize;
+  @include unselectable;
+
+  display: none; /* this will be overridden by js */
+}
diff --git a/sass/_menu.scss b/sass/_menu.scss
index 31d0bd0..52ec758 100644
--- a/sass/_menu.scss
+++ b/sass/_menu.scss
@@ -5,6 +5,8 @@
 @import 'mixins/button';
 @import 'mixins/icons';
 
+@import 'menu-size-adjust';
+
 $menu-background-color: rgba(255, 255, 255, 0.9);
 $menu-apply-btn-height: 10%;