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BPLCON1 scroll fills the display window's left edge from the bitplane shifter. On a line whose predecessor also fetched bitplanes the shifter still holds the previous line's tail, so the scrolled-in pixels are real content. At the first line of a bitplane-DMA block the line above was border, so the shifter has no carried-over data: the scroll must not pull the leading DDF pre-fetch words (already clocked out into the left border by the time the window opens) back into view. Treat fetch positions before native_x_offset as background on a block-start line. Regression example: the Kickstart 3.1 insert-disk screen leaves an uninitialised word at the bitplane base and uses a narrow DIWSTRT, which scrolled a stray fleck into the top-left corner of the first display line.
Treat SPRxPT rewrites for inactive, pre-VSTART sprite streams as seeding the next descriptor fetch, preventing descriptor words from being rendered as sprite data. Keep the existing active-stream data-origin behavior, update the low-resolution DDFSTOP granularity model, and preserve IRQ recognition latency for newly exposed non-PORTS latches. Document the sprite DMA pointer behavior and update the long-term diagnostics wording.
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Fixes some regressions with the Eon demo, and also fixes #17.