---

name: pythonista-nicegui description: NiceGUI development best practices for Python web UIs. Use when building UI components, fixing layout issues, or working with NiceGUI/Quasar/Tailwind. Triggers on "nicegui", "quasar", "tailwind", "ui.row", "ui.column", "ui.card", "ui.dialog", "gap", "spacing", "layout", "modal", "component", "styling", "flexbox", "chart", or when creating/editing UI code.

NiceGUI Development Best Practices

Core Philosophy

Understand the user workflow before building. Same data MUST look the same everywhere. Extract business logic from UI into testable controllers.

Critical Rules - Gap Spacing

NEVER use Tailwind gap-* classes in NiceGUI. ALWAYS use inline styles.

Copy# CORRECT - inline style for gap
with ui.row().classes("items-center").style("gap: 0.75rem"):
    ui.icon("info")
    ui.label("Message")

# WRONG - breaks on Ubuntu (NiceGUI bug #2171)
with ui.row().classes("items-center gap-3"):
    ui.icon("info")
    ui.label("Message")

Gap Conversion Table

Critical Rules - Height Issues

NEVER use height: 100% inside a container with only max-height.

Copy# WRONG - causes collapse to 0 height
with ui.card().style("max-height: 80vh"):
    with ui.scroll_area().style("height: 100%"):  # Collapses!
        ui.label("Content")

# CORRECT - use explicit height
with ui.card().style("height: 80vh"):
    with ui.scroll_area().style("height: 100%"):
        ui.label("Content")

Critical Rules - Side-by-Side Layouts

Use min-width: 0 on flex children when using charts or wide content.

Copy# WRONG - panels stack vertically
with ui.row().classes("w-full"):
    with ui.column().classes("flex-1"):
        ui.highchart(options)  # Forces expansion
    with ui.column().classes("flex-1"):
        ui.highchart(options)  # Wraps to next line

# CORRECT - explicit flexbox with min-width: 0
with ui.element("div").style("display: flex; width: 100%; gap: 24px"):
    with ui.element("div").style("flex: 1; min-width: 0"):
        ui.highchart(options)  # Shrinks to fit
    with ui.element("div").style("flex: 1; min-width: 0"):
        ui.highchart(options)  # Side by side

Modal/Dialog Button Docking

Primary action buttons MUST be always visible - dock to bottom.

Copywith ui.dialog() as dialog, ui.card().style(
    "height: 85vh; display: flex; flex-direction: column;"
):
    # Scrollable content
    with ui.scroll_area().style("flex: 1; overflow-y: auto;"):
        # ... form content ...

    # Sticky bottom action bar
    with ui.element("div").style(
        "position: sticky; bottom: 0; padding: 1rem; "
        "border-top: 1px solid var(--border-color);"
    ):
        with ui.row().classes("justify-end").style("gap: 0.5rem"):
            ui.button("Cancel", on_click=dialog.close)
            ui.button("Save", on_click=save_handler)

Product Thinking - Component Design

Before building UI that displays data:

1. Where else does this data type appear?
2. Should this be ONE component with modes?
3. Can user navigate from reference to source?

Copy# CORRECT - Create reusable component with modes
class PromptReference:
    def __init__(self, prompt, mode: Literal["LIBRARY", "REFERENCE", "PREVIEW"]):
        if mode == "LIBRARY":
            # Full card with edit actions
        elif mode == "REFERENCE":
            # Compact with navigation to source

# WRONG - Copy-paste UI code
# If same data in 2+ places, extract component

UI Architecture - Extract Controllers

Extract business logic from UI handlers into testable controllers.

Copy# CORRECT - Controller handles business logic
class PageController:
    async def handle_task_change(self, new_task: str) -> PageUpdate:
        data = await self.fetcher.fetch_data(new_task)
        return PageUpdate.refresh_all(data)

# UI layer is thin
async def on_task_change(e):
    logger.info(f"User selected task: {e.value}")
    update = await controller.handle_task_change(e.value)
    apply_ui_update(update)

# WRONG - Business logic in UI handlers
async def on_task_change(e):
    overview = await service.get_overview()  # Fetching in UI!
    if overview.tasks:
        selected = overview.tasks[0]  # Logic in UI!
    chart.refresh(...)  # All mixed together

Safe Quasar/Tailwind Classes

These work correctly - no inline style needed:

• Padding: q-pa-md, q-pa-lg, q-pa-sm
• Margin: q-mb-md, q-mt-md
• Width: w-full, w-1/2, w-auto
• Flexbox: items-center, justify-between, flex-row
• Colors: bg-grey-9, text-positive
• Typography: text-h5, text-body1, font-bold

Requires inline style:

• Gap spacing: gap-* -> style("gap: Xrem")

Checklists

NiceGUI Styling Checklist

• No gap-* Tailwind classes (use inline style)
• No height: 100% inside max-height container
• Side-by-side layouts use min-width: 0 on flex children
• Modal buttons are docked (always visible)

Data Display Component Checklist

• Listed ALL locations where this data appears
• Designed component with modes for each context
• User can navigate from reference to source
• Same icon, typography, color coding everywhere

UI Architecture Checklist

• Business logic in controller, not UI handlers
• Data fetching returns Pydantic models
• All user actions logged at start of handlers
• UI feedback when actions deferred/blocked

Reference Files

For detailed patterns:

• references/nicegui-styling.md - Gap, height, flexbox patterns
• references/product-thinking.md - Component design, data display
• references/ui-architecture.md - Controllers, state management

Related Skills

• For testing UI code, see /pythonista-testing
• For type safety, see /pythonista-typing
• For pattern discovery, see /pythonista-patterning