Fix notebook sticky scroll headers appearing too late when scrolling

[Copilot]

Problem

When scrolling through a notebook, sticky scroll headers would appear too late or not at all. For example, a header in cell 0 would not be revealed as sticky until much later when scrolling through content under that header.

Root Cause

The computeContent function in notebookEditorStickyScroll.ts was limited to searching for headers only within the visible range of cells. It would start looking from visibleRange.start - 1 and search forward, missing headers that were far above the viewport but still relevant to the current section.

Solution

Modified the algorithm to search backwards from the first visible cell (visibleRange.start) to find the most recent header, instead of searching forward from visibleRange.start - 1.

Before:

// Only looked from visible range backwards by 1
const startIndex = visibleRange.start - 1;
for (let currentIndex = startIndex; currentIndex < visibleRange.end; currentIndex++) {
    // Would miss headers far above viewport
}

After:

// Search backwards from first visible cell to find relevant headers
for (let i = visibleRange.start; i >= 0; i--) {
    const candidateEntry = NotebookStickyScroll.getVisibleOutlineEntry(i, notebookOutlineEntries);
    if (candidateEntry && candidateEntry.level < 7) {
        cellEntry = candidateEntry;
        break;
    }
}

Testing

Added test8 that reproduces the scenario where a header at cell 0 should be sticky when viewing cells 4-7, ensuring headers far from the viewport are properly detected.

The fix is minimal and focused - it reduces code complexity (6 additions, 32 deletions) while addressing the core issue.

Fixes #251415.

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