PDF preview intermittently ignores mouse-wheel scrolling #251

Problem

In some scenarios, a PDF preview cannot be scrolled vertically with the mouse wheel. The same preview remains scrollable by dragging its scrollbar, so the document and scroll container are functional but wheel input is not reaching or affecting it.

Expected behavior

When the pointer is over a scrollable PDF preview, mouse-wheel input should scroll the preview vertically in all supported Canvas/view states without unintended Canvas pan/zoom or parent gesture interception.

Investigation boundary

Reproduce and identify the affected states (selection, focus, zoom level, expanded/embedded preview, overlays, Canvas interaction mode, browser/platform as relevant). Trace wheel-event ownership and propagation across the PDF renderer, preview container, Canvas gesture handlers, and overlays. Distinguish an event-routing issue from PDF renderer state or CSS overflow behavior before proposing a fix.

Acceptance direction

• Mouse-wheel scrolling works wherever scrollbar dragging already works.
• Canvas pan/zoom behavior outside the PDF preview remains unchanged.
• Nested scrolling and boundary behavior are explicit and covered by focused regression tests.