term: compute the OSK view offset from the cursor row (keep input visible)

A fixed offset of OSK_ROWS pushed the cursor off the top of the screen when
there was little backlog (e.g. a fresh terminal: cursor at logical row 0 ->
screen row -3, hidden). Now the offset is max(0, wCurRow - 14): 0 when the
cursor is within the visible area, growing only enough to keep the cursor
line at the bottom visible row (14) once the terminal has filled past it.

compute_offset (from wOskVisible + wCurRow) runs at the top of
term_write_tilemap; cursor_down re-runs the tilemap when the cursor changes
rows while the OSK is up; osk_toggle just re-runs it. Shared OSK_DOCK
constant in gbos.inc keeps term.asm and osk.asm in sync.

Verified: fresh terminal + OSK shows the input at row 0; filling past row 14
shifts the view to hold the input at row 14; full screen + OSK puts it at
row 14; toggle on+off is still lossless.

1 files changed, 29 insertions, 9 deletions

diff --git a/src/term.asm b/src/term.asm
index fb6e870..64e45aa 100644
--- a/src/term.asm
+++ b/src/term.asm
@@ -80,10 +80,10 @@ term_init::
     xor a
     ld [wCurRow], a
     ld [wCurCol], a
+    ld [wViewTop], a            ; view starts unshifted (all 18 rows)
+    ld [wOskVisible], a         ; OSK hidden until toggled
     ld a, 1
     ld [wCursorOn], a
-    xor a
-    ld [wViewTop], a            ; view starts unshifted (all 18 rows)
 
     ; clear the buffer (all slots) to spaces
     ld hl, TERM_BUF
@@ -108,6 +108,7 @@ term_init::
 ; line-slot tiles. Pass 1 = indices (bank 0), pass 2 = attributes (bank 1).
 ; -----------------------------------------------------------------------------
 term_write_tilemap::
+    call compute_offset
     xor a
     ld [rVBK], a
     ld c, 0                     ; screen row
@@ -494,18 +495,37 @@ cursor_down:
     inc a
     cp TROWS
     jr c, .ok
-    call term_scroll
+    call term_scroll            ; rewrites the tilemap (offset recomputed)
     ld a, TROWS-1
-.ok
     ld [wCurRow], a
     ret
+.ok
+    ld [wCurRow], a
+    ; the cursor changed rows; if the OSK is up the view offset may need to move
+    ld a, [wOskVisible]
+    or a
+    ret z
+    jp term_write_tilemap
 
-; term_set_view - A = view-top offset. screen row -> logical row + A. 0 shows all
-; 18 rows; OSK_ROWS shifts the view up so the cursor line clears the keyboard.
-; No content is scrolled/lost - just remapped.
-term_set_view::
+; compute_offset - set wViewTop so the cursor line is always on screen: 0 when
+; the OSK is hidden; otherwise max(0, wCurRow - 14) so the cursor sits at the
+; bottom visible row (14) once the terminal has filled past it, but stays put
+; while there is little content (no shifting the cursor off the top).
+compute_offset::
+    ld a, [wOskVisible]
+    or a
+    jr nz, .osk
+    xor a
     ld [wViewTop], a
-    jp term_write_tilemap
+    ret
+.osk
+    ld a, [wCurRow]
+    sub TROWS - OSK_DOCK - 1     ; wCurRow - 14
+    jr nc, .store
+    xor a                        ; cursor above row 14 -> no shift
+.store
+    ld [wViewTop], a
+    ret
 
 ; -----------------------------------------------------------------------------
 ; term_putc - A = character. Print at the cursor, advance, handle \n \r \b.