Fix checkOutputInputFocus to support Shadow DOM traversal in notebook outputs

[Copilot]

fix microsoft/vscode-jupyter#16660

The checkOutputInputFocus function in notebook webview preloads was unable to correctly detect focused input elements within Shadow DOMs. When an input element inside a shadow root receives focus, document.activeElement only returns the shadow host element, not the actual focused element within the shadow DOM.

This caused issues with notebook outputs that render content inside shadow roots, where focus detection for input elements would fail, preventing proper input focus state tracking.

Changes

Enhanced the checkOutputInputFocus function to traverse shadow DOM boundaries:

// Before
const activeElement = window.document.activeElement;
if (!activeElement) {
    return;
}

// After  
let activeElement = window.document.activeElement;
if (!activeElement) {
    return;
}

// Check for shadow root
if (activeElement.shadowRoot) {
    activeElement = activeElement.shadowRoot.activeElement;
}

if (!activeElement) {
    return;
}

The implementation:

1. Changes activeElement from const to let to allow reassignment
2. Checks if the active element has a shadow root
3. If so, gets the actual focused element from within the shadow DOM
4. Includes proper null safety for cases where shadow root exists but no element is focused within

This follows the same pattern used elsewhere in the codebase for shadow DOM traversal and ensures accurate focus detection for inputs encapsulated in shadow roots.

Fixes focus detection for notebook output cells that use shadow DOM for rendering, improving the overall user experience when interacting with complex notebook outputs.

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[amunger]

this is already checked below, the shadow root check could just be changed to activeElement?.shadowRoot and it will follow through to the same undefined check that is handled here.

[Copilot]

Good suggestion! I've simplified the code by using optional chaining (activeElement?.shadowRoot) and removing the redundant null check. The existing check below handles both cases. Change implemented in commit d1d0bdd.