Fix scroll progress bar showing 100% with no overflow

[mattgperry]

Summary

• Fixes scroll progress immediately reporting 100% when a scroll container has no overflow (scrollHeight === clientHeight)
• The progress() utility returns 1 for a zero-width range (progress(0, 0, 0) === 1), which is correct for animations (zero-duration = complete) but wrong for scroll (no overflow = nothing scrolled)
• Added a guard in updateAxisInfo to return 0 progress when scrollLength is 0

Fixes #2950

Test plan

• Added unit test: "Reports zero progress when container has no scroll overflow"
• Verified test fails without the fix (progress returns 1 → 100%)
• Verified test passes with the fix
• All existing scroll tests pass (17/17)
• Full yarn test passes
• yarn build succeeds

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

This PR fixes a bug where a scroll progress bar would immediately report 100% progress when a scroll container has no overflow (scrollHeight === clientHeight). The root cause was that the shared progress() utility intentionally returns 1 for a zero-width range (semantically: "complete instantly"), which is correct for animations but wrong for scroll tracking where no scrollable area means nothing has been scrolled.

Key changes:

• info.ts: A one-line guard in updateAxisInfo short-circuits to 0 when scrollLength === 0, bypassing the progress(0, 0, 0) === 1 behavior.
• index.test.ts: A new unit test sets clientHeight === scrollHeight (zero overflow) and asserts that scrollLength and progress are both 0.

Minor observation:

• The new test only asserts the y-axis values — the x-axis code path (same guard, same function) is not covered by the new assertion, though the fix applies to both axes symmetrically.

Confidence Score: 5/5

• This PR is safe to merge — the fix is minimal, correct, and well-tested.
• The change is a single-line guard with no side effects on the existing scroll tracking logic. The progress utility's behavior for zero-ranges is well-understood, and the guard correctly special-cases the scroll context. The new test validates the fix, all existing 17 scroll tests continue to pass, and the build succeeds.
• No files require special attention.

Important Files Changed

Filename	Overview
packages/framer-motion/src/render/dom/scroll/info.ts	Single-line guard added to updateAxisInfo to return 0 progress when scrollLength is 0, correctly preventing the progress utility's zero-range behavior (progress(0,0,0) === 1) from propagating to scroll consumers.
packages/framer-motion/src/render/dom/scroll/tests/index.test.ts	New test verifies zero progress when container has no scroll overflow. Only the y-axis is asserted; the x-axis fix in updateAxisInfo is untested. Test follows the established new Promise<void>(async (resolve) => {...}) pattern already used 14+ times in this file.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[updateAxisInfo called] --> B["Compute scrollLength\n(scrollHeight - clientHeight)"]
    B --> C{scrollLength === 0?}
    C -- "Yes (no overflow)" --> D["axis.progress = 0\n✅ Bug fixed"]
    C -- "No (has overflow)" --> E["axis.progress = progress(0, scrollLength, current)\n✅ Normal path"]
    E --> F["progress utility\nreturns value / scrollLength"]
    D --> G[Scroll info emitted to consumer]
    F --> G

    style D fill:#d4edda,stroke:#28a745
    style C fill:#fff3cd,stroke:#ffc107

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_{Last reviewed commit: c404206}

[greptile-apps]

X-axis progress not asserted

The fix in updateAxisInfo applies to both the x and y axes, but the test only asserts latest.y. Consider also asserting latest.x.scrollLength and latest.x.progress to give symmetric coverage of the guard:

Suggested change

	expect(latest.y.scrollLength).toEqual(0)
	expect(latest.y.progress).toEqual(0)
	expect(latest.y.scrollLength).toEqual(0)
	expect(latest.y.progress).toEqual(0)
	expect(latest.x.scrollLength).toEqual(0)
	expect(latest.x.progress).toEqual(0)