Add `Get Viewport Intersection Ratio` keyword to assert element visibility in the viewport

[Snooz82]

Use case

"Is this element actually visible in the viewport right now?" is a common check for lazy-loading images, sticky headers, infinite scrolling, cookie banners and scroll-into-view behavior. The library's element state keywords answer attached/visible in the CSS sense, but not whether the element is inside the current viewport, and certainly not how much of it is.

Playwright covers this with toBeInViewport({ ratio }), backed by IntersectionObserver. Browser library has no equivalent: users today must hand-roll an Evaluate JavaScript snippet with an IntersectionObserver, a Promise and manual polling — boilerplate that is easy to get wrong and gets copy-pasted between suites.

A float getter fits the assertion-engine model perfectly and is strictly more expressive than a boolean keyword: > 0 reproduces toBeInViewport, == 0 reproduces not.toBeInViewport, and >= 0.5 covers the ratio option that a boolean never could.

Proposed keyword / arguments

Get Viewport Intersection Ratio    selector    assertion_operator=None    assertion_expected=None    message=None

Argument	Description
selector	Element selector, strict mode applies.
assertion_operator / assertion_expected / message	Standard assertion-engine arguments with auto-retry; numeric operators (==, >, >=, <, <=) apply.

Returns a float between 0.0 (fully outside the viewport) and 1.0 (fully inside), the IntersectionObserver intersection ratio of the element versus the viewport.

*** Test Cases ***
Hero Is Visible After Load
    New Page    ${URL}
    Get Viewport Intersection Ratio    id=hero    >    0

Call To Action Is At Least Half Visible
    Scroll To Element    id=cta
    Get Viewport Intersection Ratio    id=cta    >=    0.5

Footer Is Not In Viewport Initially
    Get Viewport Intersection Ratio    css=footer    ==    0
    ${ratio} =    Get Viewport Intersection Ratio    id=banner

With Set Retry Assertions For, the assertion auto-retries — e.g. waiting until a lazily loaded element scrolls into view.

Playwright API

• expect(locator).toBeInViewport (including its ratio option)

The wrapper computes the same IntersectionObserver-based ratio Playwright's assertion uses.

Implementation notes

• protobuf/playwright.proto: new selector-in / float-out RPC (or reuse an existing generic element-request shape).
• node/playwright-wrapper: resolve the locator and evaluate the intersection ratio via Playwright's injected utilities (same mechanism as toBeInViewport).
• Browser/keywords/getters.py: new keyword with @with_assertion_polling and float_str_verify_assertion-style numeric assertion, tags Getter, Assertion, PageContent.
• Docs + inv build stub regen, atest with a long scrollable page (element out of view, partially in view, fully in view).

Backwards compatibility

Purely additive: a new keyword, no changes to any existing keyword.