diff --git a/spec/dock-spec.js b/spec/dock-spec.js
index 8fda3900e..e5e26fd3a 100644
--- a/spec/dock-spec.js
+++ b/spec/dock-spec.js
@@ -120,4 +120,123 @@ describe('Dock', () => {
       })
     })
   })
+
+  describe('when you add an item to an empty dock', () => {
+    describe('when the item has a preferred size', () => {
+      it('is takes the preferred size of the item', async () => {
+        jasmine.attachToDOM(atom.workspace.getElement())
+
+        const createItem = preferredWidth => ({
+          element: document.createElement('div'),
+          getDefaultLocation() { return 'left' },
+          getPreferredWidth() { return preferredWidth }
+        })
+
+        const dock = atom.workspace.getLeftDock()
+        const dockElement = dock.getElement()
+        expect(dock.getPaneItems()).toHaveLength(0)
+
+        const item1 = createItem(111)
+        await atom.workspace.open(item1)
+
+        // It should update the width every time we go from 0 -> 1 items, not just the first.
+        expect(dock.isVisible()).toBe(true)
+        expect(dockElement.offsetWidth).toBe(111)
+        dock.destroyActivePane()
+        expect(dock.getPaneItems()).toHaveLength(0)
+        expect(dock.isVisible()).toBe(false)
+        const item2 = createItem(222)
+        await atom.workspace.open(item2)
+        expect(dock.isVisible()).toBe(true)
+        expect(dockElement.offsetWidth).toBe(222)
+
+        // Adding a second shouldn't change the size.
+        const item3 = createItem(333)
+        await atom.workspace.open(item3)
+        expect(dockElement.offsetWidth).toBe(222)
+      })
+    })
+
+    describe('when the item has no preferred size', () => {
+      it('is still has an explicit size', async () => {
+        jasmine.attachToDOM(atom.workspace.getElement())
+
+        const item = {
+          element: document.createElement('div'),
+          getDefaultLocation() { return 'left' }
+        }
+        const dock = atom.workspace.getLeftDock()
+        expect(dock.getPaneItems()).toHaveLength(0)
+
+        expect(dock.state.size).toBe(null)
+        await atom.workspace.open(item)
+        expect(dock.state.size).not.toBe(null)
+      })
+    })
+  })
+
+  describe('a deserialized dock', () => {
+    it('restores the serialized size', async () => {
+      jasmine.attachToDOM(atom.workspace.getElement())
+
+      const item = {
+        element: document.createElement('div'),
+        getDefaultLocation() { return 'left' },
+        getPreferredWidth() { return 122 },
+        serialize: () => ({deserializer: 'DockTestItem'})
+      }
+      const itemDeserializer = atom.deserializers.add({
+        name: 'DockTestItem',
+        deserialize: () => item
+      })
+      const dock = atom.workspace.getLeftDock()
+      const dockElement = dock.getElement()
+
+      await atom.workspace.open(item)
+      dock.setState({size: 150})
+      expect(dockElement.offsetWidth).toBe(150)
+      const serialized = dock.serialize()
+      dock.setState({size: 122})
+      expect(dockElement.offsetWidth).toBe(122)
+      dock.destroyActivePane()
+      dock.deserialize(serialized, atom.deserializers)
+      expect(dockElement.offsetWidth).toBe(150)
+    })
+
+    it("isn't visible if it has no items", async () => {
+      jasmine.attachToDOM(atom.workspace.getElement())
+
+      const item = {
+        element: document.createElement('div'),
+        getDefaultLocation() { return 'left' },
+        getPreferredWidth() { return 122 }
+      }
+      const dock = atom.workspace.getLeftDock()
+
+      await atom.workspace.open(item)
+      expect(dock.isVisible()).toBe(true)
+      const serialized = dock.serialize()
+      dock.deserialize(serialized, atom.deserializers)
+      expect(dock.getPaneItems()).toHaveLength(0)
+      expect(dock.isVisible()).toBe(false)
+    })
+  })
+
+  describe('when dragging an item over an empty dock', () => {
+    it('has the preferred size of the item', () => {
+      jasmine.attachToDOM(atom.workspace.getElement())
+
+      const item = {
+        element: document.createElement('div'),
+        getDefaultLocation() { return 'left' },
+        getPreferredWidth() { return 144 },
+        serialize: () => ({deserializer: 'DockTestItem'})
+      }
+      const dock = atom.workspace.getLeftDock()
+      const dockElement = dock.getElement()
+
+      dock.setDraggingItem(item)
+      expect(dock.wrapperElement.offsetWidth).toBe(144)
+    })
+  })
 })
diff --git a/src/dock.js b/src/dock.js
index f6a7aa0e4..4d9232a31 100644
--- a/src/dock.js
+++ b/src/dock.js
@@ -46,6 +46,7 @@ module.exports = class Dock {
     })
 
     this.state = {
+      size: null,
       visible: false,
       shouldAnimate: false
     }
@@ -56,6 +57,7 @@ module.exports = class Dock {
         this.didActivate(this)
       }),
       this.paneContainer.observePanes(pane => {
+        pane.onDidAddItem(this.handleDidAddPaneItem.bind(this))
         pane.onDidRemoveItem(this.handleDidRemovePaneItem.bind(this))
       }),
       this.paneContainer.onDidChangeActivePane((item) => params.didChangeActivePane(this, item)),
@@ -206,7 +208,11 @@ module.exports = class Dock {
     }
 
     const shouldBeVisible = state.visible || state.showDropTarget
-    const size = Math.max(MINIMUM_SIZE, state.size == null ? this.getInitialSize() : state.size)
+    const size = Math.max(MINIMUM_SIZE,
+      state.size ||
+      (state.draggingItem && getPreferredSize(state.draggingItem, this.location)) ||
+      DEFAULT_INITIAL_SIZE
+    )
 
     // We need to change the size of the mask...
     this.maskElement.style[this.widthOrHeight] = `${shouldBeVisible ? size : 0}px`
@@ -224,10 +230,16 @@ module.exports = class Dock {
     })
   }
 
+  handleDidAddPaneItem () {
+    if (this.state.size == null) {
+      this.setState({size: this.getInitialSize()})
+    }
+  }
+
   handleDidRemovePaneItem () {
     // Hide the dock if you remove the last item.
     if (this.paneContainer.getPaneItems().length === 0) {
-      this.setState({visible: false, hovered: false})
+      this.setState({visible: false, hovered: false, size: null})
     }
   }
 
@@ -340,13 +352,12 @@ module.exports = class Dock {
   }
 
   getInitialSize () {
-    let initialSize
     // The item may not have been activated yet. If that's the case, just use the first item.
     const activePaneItem = this.paneContainer.getActivePaneItem() || this.paneContainer.getPaneItems()[0]
-    if (activePaneItem != null) {
-      initialSize = getPreferredSize(activePaneItem, this.location)
-    }
-    return initialSize == null ? DEFAULT_INITIAL_SIZE : initialSize
+    // If there are items, we should have an explicit width; if not, we shouldn't.
+    return activePaneItem
+      ? getPreferredSize(activePaneItem, this.location) || DEFAULT_INITIAL_SIZE
+      : null
   }
 
   serialize () {
@@ -361,7 +372,7 @@ module.exports = class Dock {
   deserialize (serialized, deserializerManager) {
     this.paneContainer.deserialize(serialized.paneContainer, deserializerManager)
     this.setState({
-      size: serialized.size,
+      size: serialized.size || this.getInitialSize(),
       // If no items could be deserialized, we don't want to show the dock (even if it was visible last time)
       visible: serialized.visible && (this.paneContainer.getPaneItems().length > 0)
     })