p96gfx: Screens with height less than default (e.g. 800x500 instead of 800x600 display trash at the bottom

[codewiz]

Tested with Z3660 800x600x24 in Copperline.

To Reproduce
Steps to reproduce the behavior:

1. Go to ScreenMode
2. Uncheck "Default" on Height
3. Enter a value smaller than the default
4. Click Use

Architecture

• Amiga (Copperline)

CPU

• m68k

Version
git head

[codewiz]

I can also repro with Amiberry and the ZZ9000.card driver:

[codewiz]

The board is scanning out past the end of the framebuffer.

P96GFXBitmap__AllocBM() sizes the bitmap from its own height:

bm->memsize = (bm->bytesperline * bm->height + 7) & ~7;

so a 500 line screen gets 500 lines of VRAM. Show then programs the mode
from the sync rather than from the bitmap - SetGC(cid, modeinfo, 0) with
the sync's HDisp/VDisp - and pans at that buffer, so the board reads the
mode's full 600 lines. Everything past line 500 is whatever VRAM happens to
follow, which is why the trash changes as other bitmaps come and go. On native
hardware a screen shorter than the display just leaves the background colour
below it; p96gfx has no equivalent.

Three ways out, none of them a one-liner:

• Clearing the lines below the bitmap is not safe. That memory is not spare -
  it comes from the shared VRAM heap (Allocate(cid->vmem, bm->memsize)) and
  other bitmaps live there, so clearing past the end would corrupt them.
• Padding a displayable bitmap out to the mode's height works, but a bitmap
  can be shown in more than one mode, so it means over-allocating for the
  tallest candidate or reallocating on show - in a path that already juggles
  the VRAM/RAM fallback and the bitmap lock ordering.
• Programming a custom mode at the screen's own height looks attractive since
  the board takes an explicit VRES, but the vertical timings (VSTART, VEND,
  VMAX) come from the sync, so shrinking VRES alone would likely just produce
  a differently wrong picture.

[codewiz]

Also happens horizontally, and sizes larger than the nominal resolution don't autoscroll.

[codewiz]

In PAL mode, larger screens scroll only horizontally, not vertically.

[codewiz]

In OS3.x, the ZZ9000.card driver can open screens with non-standard sizes, as well as scroll larger screens in both directions.
I didn't test with Z3660 modes.