Ux CSS Layout Agent Skill

This skill covers CSS file organization, naming, standard sizes, programmatic layout (SplitView, Grid, scrollable), responsive patterns, and text overflow handling.

1. File Organization

CSS files are co-located with their TypeScript components:

src/vs/base/browser/ui/button/
    button.ts
    button.css
src/vs/workbench/contrib/myFeature/browser/
    myFeature.ts
    media/
        myFeature.css

Import CSS directly in the TS file:

import './media/myFeature.css';
// or for base widgets:
import './button.css';

Workbench-level global styles live in src/vs/workbench/browser/media/.

2. Class Naming

• monaco- prefix for all major components: .monaco-workbench, .monaco-split-view2, .monaco-scrollable-element
• Modifier classes: .monaco-split-view2.vertical, .monaco-split-view2.horizontal
• State classes: .visible, .focused, .active, .highlight
• Feature-specific classes use kebab-case without prefix: .my-feature, .outline-pane, .welcome-view-content

3. Standard Sizes

| Element | Size |

|---------|------|

| Part title height | 35px |

| Title padding (horizontal) | 8px |

| Title label inner padding | 12px |

| Action area padding | 5px |

| Action icon size | 16px |

| Body font-size | 13px (workbench), 11px (HTML body) |

| Line height | 1.4em |

| Validation message font-size | 12px (line-height: 17px) |

> For padding/margin/gap, border-radius, font-size/font-weight,

> codicon size and border width, prefer the design-system size tokens over

> raw px — see §10 Design-System Size Tokens.

> Canonical reference: .github/instructions/design-tokens.instructions.md

> (auto-injected for src/vs/**/*.css).

4. CSS Selector Quality

| Anti-pattern (flagged) | Correct pattern |

|------------------------|-----------------|

| ID selectors for styling (#my-widget) | Class selectors (.my-widget) |

| Overly specific selectors | Minimal specificity needed |

| Styles in the wrong file | Co-located with the component |

| Missing min-width: 0 on flex children | Prevents truncation issues |

| Forgetting pointer-events: none on hidden overlays | Prevents click-through bugs |

Never Add New !important

Do not introduce !important in new or modified CSS. When a declaration loses

the cascade, inspect the competing selector and increase specificity with the

smallest appropriate component, workbench, or state-class prefix instead.

Existing !important declarations may be preserved and must not be removed

mechanically during unrelated edits. Do not copy them, add new ones, or use

them to avoid understanding selector ownership.

The narrow exception is shared focus/active-outline suppression, where

outline: 0 !important is intentionally used to override native or global focus

indicators and prevent flashing outlines during pointer activation. Keep this

exception scoped to focus-indicator behavior; feature styling must still resolve

cascade conflicts through selector specificity.

5. SplitView Layout

File: src/vs/base/browser/ui/splitview/splitview.ts

For splitting views with draggable sashes (either horizontal or vertical):

const splitView = new SplitView(container, {
    orientation: Orientation.VERTICAL,
    proportionalLayout: true,
    styles: { separatorBorder: asCssVariable(sashBorder) }
});

// Each view implements IView:
const myView: IView = {
    element: myDomNode,
    minimumSize: 100,
    maximumSize: Number.POSITIVE_INFINITY,
    onDidChange: Event.None,
    layout(size, offset) { /* resize content */ }
};

splitView.addView(myView, Sizing.Distribute);

Use LayoutPriority.High / .Low to control which views resize first when space is constrained. Use snap: true to allow views to snap closed.

6. Grid Layout

File: src/vs/base/browser/ui/grid/grid.ts

For 2D layouts (used by editor groups):

const grid = new Grid(initialView);
grid.addView(newView, Sizing.Distribute, referenceView, Direction.Right);

7. Scrollable Content

Three classes for different needs:

| Class | When to Use |

|-------|-------------|

| SmoothScrollableElement | Animated scrolling (SplitView, ListView) |

| DomScrollableElement | Wrap existing DOM content (hovers, menus, breadcrumbs) |

| ScrollableElement | Basic single-direction scrollbar |

const scrollable = new DomScrollableElement(contentNode, {
    horizontal: ScrollbarVisibility.Auto,
    vertical: ScrollbarVisibility.Auto
});
this._register(scrollable);
container.appendChild(scrollable.getDomNode());
scrollable.scanDomNode(); // call after content changes

8. Responsive Layout

VS Code does not use CSS media queries. Instead, it uses a programmatic constraint-based layout system:

• IView.minimumSize / maximumSize — views declare their size constraints.
• SplitView and Grid distribute space according to constraints and LayoutPriority.
• ResizeObserver is used for container-aware sizing (e.g., editor auto-layout).
• The window is treated as a fixed viewport; space is distributed via sash-based resizing.

When building a responsive component:

• Set minimumSize / maximumSize appropriately.
• Use LayoutPriority.Low for panels that should collapse first.
• Use snap: true for panels that should snap closed when too small.
• Fire onDidChange when your constraints change dynamically.

9. Text Overflow & Ellipsis

All text labels that can be truncated by a resizable container must use the ellipsis pattern. Clipped text without an ellipsis is a visual bug.

Standard Ellipsis Pattern (CSS)

The three-property combo is required — all three must be present:

.my-label {
	overflow: hidden;
	white-space: nowrap;
	text-overflow: ellipsis;
}

Common Locations That Need Ellipsis

| Element | Why |

|---------|-----|

| Part title labels (h2, breadcrumbs) | Sidebar/panel can be resized narrower than the title |

| View pane header titles | View containers can be narrow |

| List/tree row labels | Rows have a fixed width from the list container |

| Tab labels (editor tabs) | Many tabs shrink to fit |

| Button labels in welcome views | Buttons have max-width constraints |

| Status bar items | Many items compete for horizontal space |

| Notification message text | Notification toast/center has fixed width |

| Tooltip/hover headings | Hovers have max-width |

| Dropdown/select items | Select boxes have bounded width |

| Badge text / descriptions | Auxiliary text in constrained columns |

Flex Container Gotchas

Flex children default to min-width: auto, which prevents text-overflow: ellipsis from working because the flex item refuses to shrink below its content width. Fix this by setting min-width: 0 on the flex child:

/* WRONG — ellipsis will NOT trigger inside a flex container */
.flex-parent {
	display: flex;
}
.flex-parent > .label {
	overflow: hidden;
	text-overflow: ellipsis;
	white-space: nowrap;
}

/* CORRECT — add min-width: 0 so the flex item can shrink */
.flex-parent > .label {
	min-width: 0;          /* ← this is the fix */
	overflow: hidden;
	text-overflow: ellipsis;
	white-space: nowrap;
}

This pattern is used throughout VS Code — for example, .monaco-icon-label-container sets min-width: 0 and flex: 1 to allow label text to truncate.

Fixed vs Flexible Elements

When a row has both fixed-size elements (icons, action buttons) and flexible text:

.row {
	display: flex;
	align-items: center;
}
.row > .icon {
	flex-shrink: 0;        /* icon never shrinks */
	width: 16px;
}
.row > .label {
	flex: 1;               /* label takes remaining space */
	min-width: 0;          /* allows shrinking below content width */
	overflow: hidden;
	text-overflow: ellipsis;
	white-space: nowrap;
}
.row > .actions {
	flex-shrink: 0;        /* action buttons never shrink */
}

This is the standard pattern for tree rows, list items, tab labels, and view pane headers.

Hover for Full Text

When text is truncated with ellipsis, the full text must be accessible via hover tooltip. Use IHoverService.setupDelayedHover() with the full untruncated text so users can read it:

this._register(this.hoverService.setupDelayedHover(labelElement, {
	content: fullLabelText,
}));

For IconLabel and list/tree renderers, this is handled automatically. For custom DOM, you must add it manually.

Anti-Patterns (NEVER DO)

• Never let text clip without an ellipsis — if overflow: hidden is set, text-overflow: ellipsis must also be set.
• Never rely on a fixed pixel width for text that could be localized — localized strings vary in length.
• Never use text-overflow: ellipsis without overflow: hidden and white-space: nowrap — all three are required.
• Never forget min-width: 0 on flex children that need to truncate.
• Never truncate text without providing a hover/tooltip for the full string.

10. Design-System Size Tokens (spacing, radius, font, codicon, stroke)

VS Code ships a design-system size ramp, registered in

src/vs/platform/theme/common/sizes/baseSizes.ts (agents font ramp in

src/vs/sessions/common/sizes.ts) and emitted as --vscode-* CSS variables.

When writing or editing CSS, prefer the token var over a raw px value wherever a

token exists. The full tables + rationale live in the auto-injected

.github/instructions/design-tokens.instructions.md (canonical source — keep

this section in sync with it). This section captures the decision logic for

deeper styling tasks.

> Every --vscode-* size var you reference must already exist in

> build/lib/stylelint/vscode-known-variables.json ("sizes" array,

> alphabetically sorted) or stylelint/hygiene fails. Adding a *new* token means

> adding it both in baseSizes.ts and that JSON file.

Spacing — padding, margin, gap

Scale (px): 0, 2, 4, 6, 8, 10, 12, 16, 20, 24, 28, 32, 36, 40 →

--vscode-spacing-sizeNone, --vscode-spacing-size20 … --vscode-spacing-size400

(token number = px × 10, so size200 = 20px).

What matters is the value, not the token. Adopting the var() is optional —

a raw px value is fine as long as it lands on the scale. What breaks rhythm is

an off-scale value (3, 5, 7, 14, 26px…). Snap off-scale values to the nearest

scale value, ties round up (5px → 6px, 3px → 4px, 1px → 2px,

26px → 28px). Each length of a shorthand is checked independently

(0 5px → 0 6px). Leave auto, %, em/rem, var()/calc() untouched.

Corner radius — border-radius

| px | Variable | Use |

|----|----------|-----|

| 2 | --vscode-cornerRadius-xSmall | very compact elements |

| 4 | --vscode-cornerRadius-small | controls (buttons, inputs) |

| 6 | --vscode-cornerRadius-medium | base / inner surfaces |

| 8 | --vscode-cornerRadius-large | prominent / outer surfaces |

| 12 | --vscode-cornerRadius-xLarge | very prominent surfaces |

| 9999 | --vscode-cornerRadius-circle | fully rounded (pills, dots) |

Snap map for off-scale literals (ties round up):

2→xSmall, 3,4→small, 5,6→medium, 7,8→large, 10,11,12→xLarge,

14,16,18,20→xLarge, 999→circle.

• Pills (radius ≈ half the element height — e.g. 28h/14r, 36h/18r,

22×22/11r) → --vscode-cornerRadius-circle, not xLarge. The

literal-nearest token would square them and lose the fully-rounded intent.

• Leave untouched: 50%, 0, 0px, inherit, any calc()/var().

Preserve !important.

Font size & weight

Generic UI ramp — pair a size token with a weight token (mirrors the

agents ramp; "Strong" = matching size token + semiBold, never a separate size):

| px | Size var | Weight |

|----|----------|--------|

| 26 | --vscode-fontSize-heading1 | semiBold |

| 18 | --vscode-fontSize-heading2 | semiBold |

| 13 | --vscode-fontSize-heading3 | semiBold |

| 13 | --vscode-fontSize-body1 | regular |

| 11 | --vscode-fontSize-body2 | regular |

| 12 | --vscode-fontSize-label1 | regular |

| 11 | --vscode-fontSize-label2 | regular |

| 10 | --vscode-fontSize-label3 | regular |

Generic weights: --vscode-fontWeight-regular (400),

--vscode-fontWeight-semiBold (600).

Deprecated — --vscode-bodyFontSize (13) → --vscode-fontSize-body1,

--vscode-bodyFontSize-small (12) → --vscode-fontSize-label1,

--vscode-bodyFontSize-xSmall (11) → --vscode-fontSize-body2.

Agents window (src/vs/sessions/**) ramp — identical values, agents--prefixed:

| px | Size var | Weight |

|----|----------|--------|

| 26 | --vscode-agents-fontSize-heading1 | semiBold |

| 18 | --vscode-agents-fontSize-heading2 | semiBold |

| 13 | --vscode-agents-fontSize-heading3 | semiBold |

| 13 | --vscode-agents-fontSize-body1 | regular |

| 11 | --vscode-agents-fontSize-body2 | regular |

| 12 | --vscode-agents-fontSize-label1 | regular |

| 11 | --vscode-agents-fontSize-label2 | regular |

| 10 | --vscode-agents-fontSize-label3 | regular |

Both weight ramps are two weights only: regular (400) and

semiBold (600) — generic --vscode-fontWeight-*, agents

--vscode-agents-fontWeight-*.

• No medium (500). font-weight: 500 is off the ramp — snap to semiBold.

Likewise 700/bold → round to the nearer of 400/600.

• "Strong" is not a separate size. "Body 1 Strong" = the matching

--vscode-fontSize-* (or --vscode-agents-fontSize-*) size token + semiBold.

Never add a strong *size*.

• normal ≡ 400 → regular. Leave inherit, lighter, bolder,

var()/calc() untouched.

Codicon size — icon font-size

Codicons are only ever 16px or 12px — never 14px or any in-between value.

| px | Variable | Use |

|----|----------|-----|

| 16 | --vscode-codiconFontSize (base) | default icon size |

| 12 | --vscode-codiconFontSize-compact | dense/inline chrome |

Compact-glyph convention: when sizing an icon at the compact 12px size, also

swap the registered glyph to its *Compact variant (e.g. Codicon.close →

Codicon.closeCompact, Codicon.add → Codicon.addCompact). CSS font-size

alone only scales the icon — it does not change to the visually-optimized

compact glyph; that requires changing the registered icon (Action2 icon: /

renderIcon). **Only swap the glyph when no CSS selector targets the original

glyph class** (e.g. .codicon-close); selectors keyed on the glyph class

(.codicon-add, .codicon-chevron-down) break when the class becomes

-compact, so update those selectors too (or size via a glyph-independent

wrapper class like .monaco-button). Some icons (settings/sliders, agent, vm,

info, lock, plus) have no compact variant — keep the regular glyph at 12px.

Stroke — border width

A single stroke thickness: 1px → --vscode-strokeThickness. Applies to the

border: 1px solid <color> shorthand and border-width: 1px. Other widths have

no token — leave them.

/* prefer */  border: var(--vscode-strokeThickness) solid var(--vscode-widget-border);
/* avoid  */  border: 1px solid var(--vscode-widget-border);

Key Files

| Area | File |

|------|------|

| SplitView | src/vs/base/browser/ui/splitview/splitview.ts |

| Grid | src/vs/base/browser/ui/grid/grid.ts |

| Scrollbar | src/vs/base/browser/ui/scrollbar/scrollableElement.ts |

| Global workbench styles | src/vs/workbench/browser/media/style.css |