diff --git a/app/components/MswBanner.tsx b/app/components/MswBanner.tsx
index 63801a29ac..f94bbaaf6c 100644
--- a/app/components/MswBanner.tsx
+++ b/app/components/MswBanner.tsx
@@ -35,7 +35,7 @@ export function MswBanner() {
   return (
     <>
       {/* The [&+*]:pt-10 style is to ensure the page container isn't pushed out of screen as it uses 100vh for layout */}
-      <label className="absolute flex h-10 w-full items-center justify-center text-sans-md text-info-secondary bg-info-secondary [&+*]:pt-10">
+      <label className="absolute z-topBar flex h-10 w-full items-center justify-center text-sans-md text-info-secondary bg-info-secondary [&+*]:pt-10">
         <Info16Icon className="mr-2" /> This is a technical preview.
         <button
           className="ml-2 flex items-center gap-0.5 text-sans-md hover:text-info"
diff --git a/app/components/ToastStack.tsx b/app/components/ToastStack.tsx
index 7ce198abd0..d98a5ac746 100644
--- a/app/components/ToastStack.tsx
+++ b/app/components/ToastStack.tsx
@@ -24,7 +24,7 @@ export function ToastStack() {
   })
 
   return (
-    <div className="pointer-events-auto fixed bottom-4 left-4 z-50 flex flex-col items-end space-y-2">
+    <div className="pointer-events-auto fixed bottom-4 left-4 z-toast flex flex-col items-end space-y-2">
       {transition((style, item) => (
         <animated.div
           style={{
diff --git a/app/components/TopBar.tsx b/app/components/TopBar.tsx
index 60280a24e6..332b080991 100644
--- a/app/components/TopBar.tsx
+++ b/app/components/TopBar.tsx
@@ -50,7 +50,7 @@ export function TopBar({ children }: { children: React.ReactNode }) {
       </div>
       {/* Height is governed by PageContainer grid */}
       {/* shrink-0 is needed to prevent getting squished by body content */}
-      <div className="border-b bg-default border-secondary">
+      <div className="z-topBar border-b bg-default border-secondary">
         <div className="mx-3 flex h-[60px] shrink-0 items-center justify-between">
           <div className="flex items-center">{otherPickers}</div>
           <div>
diff --git a/app/components/form/fields/DateTimeRangePicker.spec.tsx b/app/components/form/fields/DateTimeRangePicker.spec.tsx
index 600db3d63f..660530256a 100644
--- a/app/components/form/fields/DateTimeRangePicker.spec.tsx
+++ b/app/components/form/fields/DateTimeRangePicker.spec.tsx
@@ -7,6 +7,7 @@
  */
 import { getLocalTimeZone, now as getNow } from '@internationalized/date'
 import { fireEvent, render, screen } from '@testing-library/react'
+import ResizeObserverPolyfill from 'resize-observer-polyfill'
 import { beforeAll, describe, expect, it, vi } from 'vitest'
 
 import { clickByRole } from 'app/test/unit'
@@ -34,6 +35,8 @@ function renderLastDay() {
 }
 
 beforeAll(() => {
+  global.ResizeObserver = ResizeObserverPolyfill
+
   vi.useFakeTimers()
   vi.setSystemTime(now.toDate())
 
diff --git a/app/test/e2e/utils.ts b/app/test/e2e/utils.ts
index 0f1affbcca..527b598c19 100644
--- a/app/test/e2e/utils.ts
+++ b/app/test/e2e/utils.ts
@@ -120,3 +120,23 @@ export async function getDevUserPage(browser: Browser): Promise<Page> {
   ])
   return await browserContext.newPage()
 }
+
+/**
+ * Assert that the item is visible and in the viewport but obscured by something
+ * else, as indicated by it not being clickable. In order to avoid false
+ * positives where something is not clickable due to it being not attached or
+ * something, we assert visible and in viewport first.
+ */
+export async function expectObscured(locator: Locator) {
+  // counterintuitively, expect visible does not mean actually visible, it just
+  // means attached and not having display: none
+  await expect(locator).toBeVisible()
+  await expect(locator).toBeInViewport()
+
+  // Attempt click with `trial: true`, which means only the actionability checks
+  // run but the click does not actually happen. Short timeout means this will
+  // fail fast if not clickable.
+  await expect(
+    async () => await locator.click({ trial: true, timeout: 50 })
+  ).rejects.toThrow(/locator.click: Timeout 50ms exceeded/)
+}
diff --git a/app/test/e2e/z-index.e2e.ts b/app/test/e2e/z-index.e2e.ts
new file mode 100644
index 0000000000..180d2fae42
--- /dev/null
+++ b/app/test/e2e/z-index.e2e.ts
@@ -0,0 +1,73 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * Copyright Oxide Computer Company
+ */
+import { expect, test } from '@playwright/test'
+
+import { expectObscured } from './utils'
+
+test('Dropdown content can scroll off page and doesn’t hide TopBar', async ({ page }) => {
+  // load the page
+  await page.goto('/utilization')
+  await expect(page.getByText('Capacity & Utilization')).toBeVisible()
+
+  const button = page.getByRole('button', { name: 'All projects' })
+
+  // click on the 'All projects' dropdown
+  await button.click()
+  const options = ['All projects', 'mock-project', 'other-project']
+  for (const name of options) {
+    const option = page.getByRole('option', { name })
+    await expect(option).toBeVisible()
+    await expect(option).toBeInViewport()
+  }
+
+  // scroll the page down by 275px
+  await page.mouse.wheel(0, 275)
+
+  // if we don't do this, the test doesn't wait long enough for the following
+  // assertions to become true
+  await expect(button).not.toBeInViewport()
+
+  // now the top the listbox option is obscured by the topbar
+  await expectObscured(page.getByRole('option', { name: 'All projects' }))
+
+  // but we can still click the bottom one
+  await page.getByRole('option', { name: 'other-project' }).click()
+  await expect(page.getByRole('button', { name: 'other-project' })).toBeVisible()
+})
+
+test('Dropdown content in SidebarModal shows on screen', async ({ page }) => {
+  // go to an instance’s Network Interfaces page
+  await page.goto('/projects/mock-project/instances/db1/network-interfaces')
+
+  // stop the instance
+  await page.getByRole('button', { name: 'Instance actions' }).click()
+  await page.getByRole('menuitem', { name: 'Stop' }).click()
+
+  // open the add network interface side modal
+  await page.getByRole('button', { name: 'Add network interface' }).click()
+
+  // fill out the form
+  await page.getByLabel('Name').fill('alt-nic')
+
+  // select the VPC and subnet via the dropdowns. The fact that the options are
+  // clickable means they are not obscured due to having a too-low z-index
+  await page.getByRole('button', { name: 'VPC' }).click()
+  await page.getByRole('option', { name: 'mock-vpc' }).click()
+  await page.getByRole('button', { name: 'Subnet' }).click()
+  await page.getByRole('option', { name: 'mock-subnet' }).click()
+
+  const sidebar = page.getByRole('dialog', { name: 'Add network interface' })
+
+  // verify that the SideModal header is positioned above the TopBar
+  await expectObscured(page.getByRole('button', { name: 'User menu' }))
+
+  // test that the form can be submitted and a new network interface is created
+  await sidebar.getByRole('button', { name: 'Add network interface' }).click()
+  await expect(sidebar).toBeHidden()
+  await expect(page.getByText('alt-nic')).toBeVisible()
+})
diff --git a/libs/ui/lib/date-picker/Popover.tsx b/libs/ui/lib/date-picker/Popover.tsx
index 7373d8e931..bd361cdb31 100644
--- a/libs/ui/lib/date-picker/Popover.tsx
+++ b/libs/ui/lib/date-picker/Popover.tsx
@@ -36,7 +36,7 @@ export function Popover(props: PopoverProps) {
       <div
         {...popoverProps}
         ref={ref}
-        className="rounded-md absolute top-full z-10 mt-2 rounded-lg border bg-raise border-secondary elevation-2"
+        className="rounded-md absolute top-full z-popover mt-2 rounded-lg border bg-raise border-secondary elevation-2"
       >
         <DismissButton onDismiss={state.close} />
         {children}
diff --git a/libs/ui/lib/listbox/Listbox.tsx b/libs/ui/lib/listbox/Listbox.tsx
index 2949752cc6..1cd309d3ec 100644
--- a/libs/ui/lib/listbox/Listbox.tsx
+++ b/libs/ui/lib/listbox/Listbox.tsx
@@ -5,13 +5,22 @@
  *
  * Copyright Oxide Computer Company
  */
-import { FloatingPortal, flip, offset, size, useFloating } from '@floating-ui/react'
+import {
+  FloatingPortal,
+  autoUpdate,
+  flip,
+  offset,
+  size,
+  useFloating,
+} from '@floating-ui/react'
 import { Listbox as Select } from '@headlessui/react'
 import cn from 'classnames'
 import type { ReactNode } from 'react'
 
-import { FieldLabel, SpinnerLoader, TextInputHint } from '@oxide/ui'
-import { SelectArrows6Icon } from '@oxide/ui'
+import { FieldLabel, SelectArrows6Icon, SpinnerLoader, TextInputHint } from '@oxide/ui'
+
+import { useIsInModal } from '../modal/Modal'
+import { useIsInSideModal } from '../side-modal/SideModal'
 
 export type ListboxItem<Value extends string = string> = {
   value: Value
@@ -68,11 +77,19 @@ export const Listbox = <Value extends string = string>({
         },
       }),
     ],
+    whileElementsMounted: autoUpdate,
   })
 
   const selectedItem = selected && items.find((i) => i.value === selected)
   const noItems = !isLoading && items.length === 0
   const isDisabled = disabled || noItems
+  const isInModal = useIsInModal()
+  const isInSideModal = useIsInSideModal()
+  const zIndex = isInModal
+    ? 'z-modalDropdown'
+    : isInSideModal
+    ? 'z-sideModalDropdown'
+    : 'z-contentDropdown'
 
   return (
     <div className={cn('relative', className)}>
@@ -134,7 +151,7 @@ export const Listbox = <Value extends string = string>({
               <Select.Options
                 ref={refs.setFloating}
                 style={floatingStyles}
-                className="ox-menu pointer-events-auto z-50 overflow-y-auto !outline-none"
+                className={`ox-menu pointer-events-auto ${zIndex} overflow-y-auto !outline-none`}
               >
                 {items.map((item) => (
                   <Select.Option
diff --git a/libs/ui/lib/modal/Modal.tsx b/libs/ui/lib/modal/Modal.tsx
index 657561e6ad..12cd5e7733 100644
--- a/libs/ui/lib/modal/Modal.tsx
+++ b/libs/ui/lib/modal/Modal.tsx
@@ -16,9 +16,7 @@ import { Close12Icon } from '../icons'
 
 const ModalContext = createContext(false)
 
-export const useIsInModal = () => {
-  return useContext(ModalContext)
-}
+export const useIsInModal = () => useContext(ModalContext)
 
 export type ModalProps = {
   title?: string
@@ -63,7 +61,7 @@ export function Modal({ children, onDismiss, title, isOpen }: ModalProps) {
                   aria-hidden
                 />
                 <AnimatedDialogContent
-                  className="DialogContent ox-modal pointer-events-auto fixed left-1/2 top-1/2 z-40 m-0 flex max-h-[min(800px,80vh)] w-auto min-w-[28rem] max-w-[32rem] flex-col justify-between rounded-lg border p-0 bg-raise border-secondary elevation-2"
+                  className="DialogContent ox-modal pointer-events-auto fixed left-1/2 top-1/2 z-modal m-0 flex max-h-[min(800px,80vh)] w-auto min-w-[28rem] max-w-[32rem] flex-col justify-between rounded-lg border p-0 bg-raise border-secondary elevation-2"
                   aria-labelledby={titleId}
                   style={{
                     transform: y.to((value) => `translate3d(-50%, ${-50 + value}%, 0px)`),
diff --git a/libs/ui/lib/side-modal/SideModal.tsx b/libs/ui/lib/side-modal/SideModal.tsx
index f546230cf5..16e795f872 100644
--- a/libs/ui/lib/side-modal/SideModal.tsx
+++ b/libs/ui/lib/side-modal/SideModal.tsx
@@ -8,7 +8,7 @@
 import * as Dialog from '@radix-ui/react-dialog'
 import { animated, useTransition } from '@react-spring/web'
 import cn from 'classnames'
-import React, { type ReactNode, useRef } from 'react'
+import React, { type ReactNode, createContext, useContext, useRef } from 'react'
 
 import { Message } from '@oxide/ui'
 import { classed } from '@oxide/util'
@@ -18,6 +18,10 @@ import { useIsOverflow } from 'app/hooks'
 import { Close12Icon, Error12Icon } from '../icons'
 import './side-modal.css'
 
+const SideModalContext = createContext(false)
+
+export const useIsInSideModal = () => useContext(SideModalContext)
+
 export type SideModalProps = {
   title?: string
   subtitle?: ReactNode
@@ -53,65 +57,69 @@ export function SideModal({
     config: isOpen && animate ? config : { duration: 0 },
   })
 
-  return transitions(
-    ({ x }, item) =>
-      item && (
-        <Dialog.Root
-          open
-          onOpenChange={(open) => {
-            if (!open) onDismiss()
-          }}
-          // https://github.com/radix-ui/primitives/issues/1159#issuecomment-1559813266
-          modal={false}
-        >
-          <Dialog.Portal>
-            <div
-              className="DialogOverlay pointer-events-auto"
-              onClick={onDismiss}
-              aria-hidden
-            />
-            <AnimatedDialogContent
-              className="DialogContent ox-side-modal pointer-events-auto fixed bottom-0 right-0 top-0 m-0 flex w-[32rem] flex-col justify-between border-l p-0 bg-raise border-secondary elevation-2"
-              aria-labelledby={titleId}
-              style={{
-                transform: x.to((value) => `translate3d(${value}%, 0px, 0px)`),
+  return (
+    <SideModalContext.Provider value>
+      {transitions(
+        ({ x }, item) =>
+          item && (
+            <Dialog.Root
+              open
+              onOpenChange={(open) => {
+                if (!open) onDismiss()
               }}
+              // https://github.com/radix-ui/primitives/issues/1159#issuecomment-1559813266
+              modal={false}
             >
-              {title && (
-                <Dialog.Title asChild>
-                  <>
-                    <SideModal.Title id={titleId} title={title} subtitle={subtitle} />
-
-                    {errors && errors.length > 0 && (
-                      <div className="mb-6">
-                        <Message
-                          variant="error"
-                          content={
-                            errors.length === 1 ? (
-                              errors[0]
-                            ) : (
-                              <>
-                                <div>{errors.length} issues:</div>
-                                <ul className="ml-4 list-disc">
-                                  {errors.map((error, idx) => (
-                                    <li key={idx}>{error}</li>
-                                  ))}
-                                </ul>
-                              </>
-                            )
-                          }
-                          title={errors.length > 1 ? 'Errors' : 'Error'}
-                        />
-                      </div>
-                    )}
-                  </>
-                </Dialog.Title>
-              )}
-              {children}
-            </AnimatedDialogContent>
-          </Dialog.Portal>
-        </Dialog.Root>
-      )
+              <Dialog.Portal>
+                <div
+                  className="DialogOverlay pointer-events-auto"
+                  onClick={onDismiss}
+                  aria-hidden
+                />
+                <AnimatedDialogContent
+                  className="DialogContent ox-side-modal pointer-events-auto fixed bottom-0 right-0 top-0 z-sideModal m-0 flex w-[32rem] flex-col justify-between border-l p-0 bg-raise border-secondary elevation-2"
+                  aria-labelledby={titleId}
+                  style={{
+                    transform: x.to((value) => `translate3d(${value}%, 0px, 0px)`),
+                  }}
+                >
+                  {title && (
+                    <Dialog.Title asChild>
+                      <>
+                        <SideModal.Title id={titleId} title={title} subtitle={subtitle} />
+
+                        {errors && errors.length > 0 && (
+                          <div className="mb-6">
+                            <Message
+                              variant="error"
+                              content={
+                                errors.length === 1 ? (
+                                  errors[0]
+                                ) : (
+                                  <>
+                                    <div>{errors.length} issues:</div>
+                                    <ul className="ml-4 list-disc">
+                                      {errors.map((error, idx) => (
+                                        <li key={idx}>{error}</li>
+                                      ))}
+                                    </ul>
+                                  </>
+                                )
+                              }
+                              title={errors.length > 1 ? 'Errors' : 'Error'}
+                            />
+                          </div>
+                        )}
+                      </>
+                    </Dialog.Title>
+                  )}
+                  {children}
+                </AnimatedDialogContent>
+              </Dialog.Portal>
+            </Dialog.Root>
+          )
+      )}
+    </SideModalContext.Provider>
   )
 }
 
diff --git a/libs/ui/styles/components/menu-list.css b/libs/ui/styles/components/menu-list.css
index 4a39e0133c..8a9d719712 100644
--- a/libs/ui/styles/components/menu-list.css
+++ b/libs/ui/styles/components/menu-list.css
@@ -7,7 +7,7 @@
  */
 
 .ox-menu {
-  @apply z-50 max-h-[17.5rem] overflow-y-auto rounded border bg-raise border-secondary elevation-2;
+  @apply max-h-[17.5rem] overflow-y-auto rounded border bg-raise border-secondary elevation-2;
 }
 
 .ox-menu-item {
diff --git a/package-lock.json b/package-lock.json
index 3ce62c62b0..c0058f939a 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -109,6 +109,7 @@
         "postcss-pseudo-classes": "^0.2.1",
         "prettier": "^2.8.8",
         "prettier-plugin-tailwindcss": "^0.3.0",
+        "resize-observer-polyfill": "^1.5.1",
         "semver": "^7.5.2",
         "strict-event-emitter": "^0.5.0",
         "style-dictionary": "^3.1.1",
@@ -18870,6 +18871,12 @@
       "integrity": "sha512-KigOCHcocU3XODJxsu8i/j8T9tzT4adHiecwORRQ0ZZFcp7ahwXuRU1m+yuO90C5ZUyGeGfocHDI14M3L3yDAQ==",
       "devOptional": true
     },
+    "node_modules/resize-observer-polyfill": {
+      "version": "1.5.1",
+      "resolved": "https://registry.npmjs.org/resize-observer-polyfill/-/resize-observer-polyfill-1.5.1.tgz",
+      "integrity": "sha512-LwZrotdHOo12nQuZlHEmtuXdqGoOD0OhaxopaNFxWzInpEgaLWoVuAMbTzixuosCx2nEG58ngzW3vxdWoxIgdg==",
+      "dev": true
+    },
     "node_modules/resolve": {
       "version": "1.22.2",
       "resolved": "https://registry.npmjs.org/resolve/-/resolve-1.22.2.tgz",
@@ -35892,6 +35899,12 @@
       "integrity": "sha512-KigOCHcocU3XODJxsu8i/j8T9tzT4adHiecwORRQ0ZZFcp7ahwXuRU1m+yuO90C5ZUyGeGfocHDI14M3L3yDAQ==",
       "devOptional": true
     },
+    "resize-observer-polyfill": {
+      "version": "1.5.1",
+      "resolved": "https://registry.npmjs.org/resize-observer-polyfill/-/resize-observer-polyfill-1.5.1.tgz",
+      "integrity": "sha512-LwZrotdHOo12nQuZlHEmtuXdqGoOD0OhaxopaNFxWzInpEgaLWoVuAMbTzixuosCx2nEG58ngzW3vxdWoxIgdg==",
+      "dev": true
+    },
     "resolve": {
       "version": "1.22.2",
       "resolved": "https://registry.npmjs.org/resolve/-/resolve-1.22.2.tgz",
diff --git a/package.json b/package.json
index f51efc0b7b..8490fe474a 100644
--- a/package.json
+++ b/package.json
@@ -130,6 +130,7 @@
     "postcss-pseudo-classes": "^0.2.1",
     "prettier": "^2.8.8",
     "prettier-plugin-tailwindcss": "^0.3.0",
+    "resize-observer-polyfill": "^1.5.1",
     "semver": "^7.5.2",
     "strict-event-emitter": "^0.5.0",
     "style-dictionary": "^3.1.1",
diff --git a/tailwind.config.js b/tailwind.config.js
index daf25a2cdd..0aca5afb91 100644
--- a/tailwind.config.js
+++ b/tailwind.config.js
@@ -42,6 +42,17 @@ module.exports = {
         lg: { min: '1024px', max: '1279px' },
         xl: { min: '1280px', max: '1535px' },
       },
+      zIndex: {
+        toast: '50',
+        modalDropdown: '50',
+        modal: '40',
+        sideModalDropdown: '40',
+        sideModal: '30',
+        topBar: '20',
+        popover: '10',
+        contentDropdown: '10',
+        content: '0',
+      },
     },
     borderRadius: {
       none: 0,