DeskTop: Improve scrolling in directory windows. Resolves #711

The previous scroll logic for windows made each increment 1/20th of
the scroll range, which made scrolling down in windows without tons of
icons absurdly slow. There were also numerous edge cases requiring
hacky fixes, and spaghetti code throughout DeskTop to implement it.

Rework scrolling entirely. A ScrollManager scope is introduced, which
exposes a much simplified set of entry points - actions on the
scrollbar (clicks or thumb moves) and adjusting the scrollbars
following a content change or resize. Callers are also
simplified/reworked.

Specific behavior changes:

* Scroll thumbs have full fidelity (up to 256 steps).

* Scroll arrows move by roughly 1/2 the grid spacing, when in icon
  view. When in list view they scroll vertically by a single row.

* Rows when in list view are now spaced out like list controls.

* Scroll position is reset when switching from icon to list view.

This ends up saving roughly 178 bytes.

RELEASE-NOTES.md

@@ -15,6 +15,7 @@ Project Page: https://github.com/a2stuff/a2d
 * File > Copy a File... is now File > Copy To... and operates on current selection.
 * File > Delete a File... is now File > Delete and operates on current selection.
 * Support up to 14 devices. ([#712](https://github.com/a2stuff/a2d/issues/712))
+* Improve scrolling in directory windows. ([#711](https://github.com/a2stuff/a2d/issues/711))
 
 ### Disk Copy
 

desk.acc/show.text.file.s

@@ -1239,102 +1239,8 @@ window_id:      .byte   kDAWindowId
 .endproc
 
 ;;; ============================================================
-;;; 32 bit by 32 bit division with 32 bit result
-;;; Based on: https://www.reddit.com/r/asm/comments/nbu2dj/32_bit_division_subroutine_on_the_6502/gy21eog/
 
-numerator       := $10
-denominator     := $14
-quotient        := numerator
-
-.proc Div_32_32
-remainder       := $18
-temp            := $1C
-
-        ldx     #4-1
-        lda     #0
-:       sta     remainder,x
-        dex
-        bpl     :-
-
-        ldx     #32             ; bits
-divloop:
-        asl     numerator+0
-        rol     numerator+1
-        rol     numerator+2
-        rol     numerator+3
-
-        rol     remainder+0
-        rol     remainder+1
-        rol     remainder+2
-        rol     remainder+3
-
-        sec
-subtract:
-        lda     remainder+0
-        sbc     denominator+0
-        sta     temp+0
-
-        lda     remainder+1
-        sbc     denominator+1
-        sta     temp+1
-
-        lda     remainder+2
-        sbc     denominator+2
-        sta     temp+2
-
-        lda     remainder+3
-        sbc     denominator+3
-        sta     temp+3
-
-        bcc     next            ; remainder > divisor?
-        inc     numerator       ; yes
-
-        ldy     #4-1            ; remainder = temp
-:       lda     temp,y
-        sta     remainder,y
-        dey
-        bpl     :-
-
-next:   dex
-        bne     divloop
-
-        rts
-.endproc
-
-;;; ============================================================
-;;; 16 bit by 16 bit multiply with 32 bit product
-
-multiplier      := $10
-multiplicand    := $12
-product         := $14
-
-.proc Mul_16_16
-        lda     #0
-        sta     product+2
-        sta     product+3
-
-        ldx     #16
-shift:
-        lsr     multiplier+1
-        ror     multiplier
-        bcc     rotate
-        lda     product+2
-        clc
-        adc     multiplicand
-        sta     product+2
-        lda     product+3
-        adc     multiplicand+1
-rotate:
-        ror     a
-        sta     product+3
-        ror     product+2
-        ror     product+1
-        ror     product
-        dex
-        bne     shift
-
-        rts
-.endproc
+        .include "../lib/muldiv32.s"
 
 ;;; ============================================================
 

desktop/internal.inc

@@ -71,6 +71,7 @@ kWindowHeaderHeight     = 14
 
 kFirstRowBaseline       = 16
 kListViewWidth          = 410
+kListViewRowHeight      = kSystemFontHeight + 1
 
 ;;; --------------------------------------------------
 ;;; Icon views
@@ -85,16 +86,28 @@ kIconSpacingY   = 32
 
 ;;; Bounding box padding, which defines icon placing, window sizing and
 ;;; scrollbars. This does not take into account the icon's actual width
-;;; (bitmap or label size). The header is included to simplify
-;;; calculations.
+;;; (bitmap or label size).
 
+;;; TODO: Rework callers to remove need for kWindowHeaderHeight.
 kIconBBoxOffsetTop      = kWindowHeaderHeight + 4
 kIconBBoxOffsetLeft     = 45
 kIconBBoxOffsetBottom   = kIconLabelHeight + 4
 kIconBBoxOffsetRight    = 37 + kIconWidth ; accommodate icon + label
-;;; NOTE: kIconBBoxOffsetRight should probably be se to
+;;; NOTE: kIconBBoxOffsetRight should probably be set to
 ;;; kIconBBoxOffsetLeft + kIconWidth but aesthetically that looks bad.
 
+;;; --------------------------------------------------
+;;; Scrolling
+
+;;; Allow maximum fidelity
+kScrollThumbMax = $FF
+
+;;; Arbitrary - these feel good, but can be tweaked
+kIconViewScrollTickH = kIconSpacingY
+kIconViewScrollTickV = kIconSpacingY / 2
+kListViewScrollTickH = kListViewRowHeight * 2
+kListViewScrollTickV = kListViewRowHeight
+
 ;;; ============================================================
 ;;; Common Shortcuts