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fix(ui-modal): avoid redundant Modal.Body re-renders from its observers #2701

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fix(ui-modal): avoid redundant Modal.Body re-renders from its observers #2701

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53 changes: 37 additions & 16 deletions packages/ui-modal/src/Modal/v1/ModalBody/index.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -33,7 +33,7 @@ import generateStyle from './styles.js'
import generateComponentTheme from './theme.js'

import { allowedProps } from './props.js'
import type { ModalBodyProps } from './props'
import type { ModalBodyProps, ModalBodyState } from './props'
import { UIElement } from '@instructure/shared-types'
import ModalContext from '../ModalContext.js'

Expand All @@ -44,7 +44,7 @@ id: Modal.Body
---
**/
@withStyle(generateStyle, generateComponentTheme)
class ModalBody extends Component<ModalBodyProps> {
class ModalBody extends Component<ModalBodyProps, ModalBodyState> {
static displayName = 'ModalBody'
static readonly componentId = 'Modal.Body'

Expand All @@ -55,6 +55,11 @@ class ModalBody extends Component<ModalBodyProps> {
variant: 'default'
}

state: ModalBodyState = { isFirefox: false, needsTabIndex: false }
// mirrors state.needsTabIndex, but readable synchronously in the observer
// callbacks below, where a pending setState would make this.state stale
private lastNeedsTabIndex = false

ref: UIElement | null = null
private resizeObserver?: ResizeObserver
private mutationObserver?: MutationObserver
Expand Down Expand Up @@ -92,9 +97,10 @@ class ModalBody extends Component<ModalBodyProps> {

const finalRef = this.getFinalRef(this.ref)
if (finalRef && typeof ResizeObserver !== 'undefined') {
this.resizeObserver = new ResizeObserver(() => this.forceUpdate())
this.syncTabIndex()
this.resizeObserver = new ResizeObserver(this.syncTabIndex)
this.resizeObserver.observe(finalRef)
this.mutationObserver = new MutationObserver(() => this.forceUpdate())
this.mutationObserver = new MutationObserver(this.syncTabIndex)
this.mutationObserver.observe(finalRef, {
childList: true,
subtree: true,
Expand Down Expand Up @@ -122,6 +128,32 @@ class ModalBody extends Component<ModalBodyProps> {
this.mutationObserver?.disconnect()
}

// The body is a tab stop only while it can be scrolled but holds nothing
// focusable. Both inputs come from the DOM, so the observers recompute them
// on resize and on subtree changes — which is most changes inside the body,
// the vast majority of them leaving the result identical. Comparing against
// the last computed value before calling setState keeps those callbacks from
// scheduling an update at all, rather than scheduling one React later
// discards: `setState` warns about updates outside `act()` in tests as soon
// as it schedules, so bailing out inside the updater would be too late.
syncTabIndex = () => {
const finalRef = this.getFinalRef(this.ref)
const hasScrollbar =
!!finalRef &&
Math.abs(
(finalRef.scrollHeight ?? 0) -
(finalRef.getBoundingClientRect()?.height ?? 0)
) > 1
const needsTabIndex = hasScrollbar && findTabbable(finalRef).length === 0

if (needsTabIndex === this.lastNeedsTabIndex) return
this.lastNeedsTabIndex = needsTabIndex
this.setState({ needsTabIndex })
}

// this recursive function is needed because `ref` can be a React component.
// TODO rethink, the 'as' prop, likely its not a good idea to allow React
// components. See INSTUI-4674
getFinalRef(el: UIElement): Element | undefined {
if (!el) {
return undefined
Expand All @@ -144,18 +176,7 @@ class ModalBody extends Component<ModalBodyProps> {
ModalBody
)
const isFit = overflow === 'fit'
// this recursive function is needed because `ref` can be a React component.
// TODO rethink, the 'as' prop, likely its not a good idea to allow React
// components. See INSTUI-4674
const finalRef = this.getFinalRef(this.ref)
const hasScrollbar =
finalRef &&
Math.abs(
(finalRef.scrollHeight ?? 0) -
(finalRef.getBoundingClientRect()?.height ?? 0)
) > 1
const hasTabbableChildren = !!finalRef && findTabbable(finalRef).length > 0
const needsTabIndex = hasScrollbar && !hasTabbableChildren
const { needsTabIndex } = this.state
return (
<ModalContext.Consumer>
{(value) => (
Expand Down
1 change: 1 addition & 0 deletions packages/ui-modal/src/Modal/v1/ModalBody/props.ts
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -56,6 +56,7 @@ type ModalBodyStyle = ComponentStyle<'modalBody'>

type ModalBodyState = {
isFirefox: boolean
needsTabIndex: boolean
}
const allowedProps: AllowedPropKeys = [
'children',
Expand Down
53 changes: 37 additions & 16 deletions packages/ui-modal/src/Modal/v2/ModalBody/index.tsx
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -32,7 +32,7 @@ import { withStyleNew } from '@instructure/emotion'
import generateStyle from './styles.js'

import { allowedProps } from './props.js'
import type { ModalBodyProps } from './props'
import type { ModalBodyProps, ModalBodyState } from './props'
import { UIElement } from '@instructure/shared-types'
import ModalContext from '../ModalContext.js'

Expand All @@ -43,7 +43,7 @@ id: Modal.Body
---
**/
@withStyleNew(generateStyle, 'ModalBody')
class ModalBody extends Component<ModalBodyProps> {
class ModalBody extends Component<ModalBodyProps, ModalBodyState> {
static displayName = 'ModalBody'
static readonly componentId = 'Modal.Body'
static readonly themeId = 'ModalBody'
Expand All @@ -55,6 +55,11 @@ class ModalBody extends Component<ModalBodyProps> {
variant: 'default'
}

state: ModalBodyState = { isFirefox: false, needsTabIndex: false }
// mirrors state.needsTabIndex, but readable synchronously in the observer
// callbacks below, where a pending setState would make this.state stale
private lastNeedsTabIndex = false

ref: UIElement | null = null
private resizeObserver?: ResizeObserver
private mutationObserver?: MutationObserver
Expand Down Expand Up @@ -92,9 +97,10 @@ class ModalBody extends Component<ModalBodyProps> {

const finalRef = this.getFinalRef(this.ref)
if (finalRef && typeof ResizeObserver !== 'undefined') {
this.resizeObserver = new ResizeObserver(() => this.forceUpdate())
this.syncTabIndex()
this.resizeObserver = new ResizeObserver(this.syncTabIndex)
this.resizeObserver.observe(finalRef)
this.mutationObserver = new MutationObserver(() => this.forceUpdate())
this.mutationObserver = new MutationObserver(this.syncTabIndex)
this.mutationObserver.observe(finalRef, {
childList: true,
subtree: true,
Expand Down Expand Up @@ -122,6 +128,32 @@ class ModalBody extends Component<ModalBodyProps> {
this.mutationObserver?.disconnect()
}

// The body is a tab stop only while it can be scrolled but holds nothing
// focusable. Both inputs come from the DOM, so the observers recompute them
// on resize and on subtree changes — which is most changes inside the body,
// the vast majority of them leaving the result identical. Comparing against
// the last computed value before calling setState keeps those callbacks from
// scheduling an update at all, rather than scheduling one React later
// discards: `setState` warns about updates outside `act()` in tests as soon
// as it schedules, so bailing out inside the updater would be too late.
Comment on lines +131 to +138

@matyasf matyasf Sep 1, 2026

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please simplify to

  // The body is a tab stop only while it can be scrolled but holds nothing
  // focusable.

syncTabIndex = () => {
const finalRef = this.getFinalRef(this.ref)
const hasScrollbar =
!!finalRef &&
Math.abs(
(finalRef.scrollHeight ?? 0) -
(finalRef.getBoundingClientRect()?.height ?? 0)
) > 1
const needsTabIndex = hasScrollbar && findTabbable(finalRef).length === 0

if (needsTabIndex === this.lastNeedsTabIndex) return
this.lastNeedsTabIndex = needsTabIndex
this.setState({ needsTabIndex })
}

// this recursive function is needed because `ref` can be a React component.
// TODO rethink, the 'as' prop, likely its not a good idea to allow React
// components. See INSTUI-4674
getFinalRef(el: UIElement): Element | undefined {
if (!el) {
return undefined
Expand Down Expand Up @@ -152,18 +184,7 @@ class ModalBody extends Component<ModalBodyProps> {
ModalBody
)
const isFit = overflow === 'fit'
// this recursive function is needed because `ref` can be a React component.
// TODO rethink, the 'as' prop, likely its not a good idea to allow React
// components. See INSTUI-4674
const finalRef = this.getFinalRef(this.ref)
const hasScrollbar =
finalRef &&
Math.abs(
(finalRef.scrollHeight ?? 0) -
(finalRef.getBoundingClientRect()?.height ?? 0)
) > 1
const hasTabbableChildren = !!finalRef && findTabbable(finalRef).length > 0
const needsTabIndex = hasScrollbar && !hasTabbableChildren
const { needsTabIndex } = this.state
return (
<ModalContext.Consumer>
{(value) => (
Expand Down
1 change: 1 addition & 0 deletions packages/ui-modal/src/Modal/v2/ModalBody/props.ts
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -62,6 +62,7 @@ type ModalBodyStyle = ComponentStyle<'modalBody'>

type ModalBodyState = {
isFirefox: boolean
needsTabIndex: boolean
}
const allowedProps: AllowedPropKeys = [
'children',
Expand Down
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