shadowfb artifacts when scrolling back.

Hi,

On Thu, Apr 09, 2026 at 12:28:12AM +0000, RVP wrote:

> There're 2 issues (both fairly minor) with enabling the shadow FB:
> [...]
> 2. Screen-artefacts on the console after paging with Shift-PgUp/PgDn.

Did you see this with UEFI? If yes, please open a PR. If no, I guess I'll
try to setup an UEFI machine to test. It turned out the work machine I
misremembered to boot in UEFI mode only uses GPT but with MBR.

Anyway, the effect I see is not erasing / overwriting the ends of lines 
properly after or when scrolling back and forth again; this smells like an
optimization that implicitly assumes only ever writing downwards.

Should be easily (ha!) to spot by staring at the code.

	-is