Improve scrolling in icon views

[inexorabletash]

In DeskTop, the scrollbars always have a thumbmax of 20 (see desktop/main.s CreateIconsForWindow) and the scroll bar arrows scroll by 1/20th of the range at a time. This means if the icon bounds are only e.g. 40 pixels larger than the viewport, each scroll tick is 2 pixels, which is really slow.

To improve this, the max should be dynamic and based on comparing the icon bbox and viewport, and then each scroll tick should be a fixed number of pixels instead.

Beware of edge cases when the icon bbox is offset from the view (e.g. potentially even smaller); the effective bounds probably need to be the bounding rect containing both the icon bbox and the viewport.

[inexorabletash]

Psuedocode for the algorithm I think we want:

Inputs:

• icon bbox (lo/hi in window space)
• viewport (lo/hi in window space)
• ubox (union of icon bbox / viewport) - necessary for edge cases
• tick (pixels, different in x and y coords, maybe 32 / 16)
• thumb.pos
• thumb.max (can be anything... use 255 for full precision?)

Preamble:

• w = vp.hi - vp.lo
• old = vp.lo

Switch on action:

• arrow inc:
  1. vp.hi += tick
  2. goto clamp_hi
• arrow dec:
  1. vp.lo -= tick;
  2. goto clamp_lo
• page inc:
  1. vp.hi += w (maybe w - tick?)
  2. goto clamp_hi
• page dec:
  1. vp.lo -= w (maybe w - tick?)
  2. goto clamp_lo
• thumb:
  1. vp.lo = ubox.lo + (ubox.hi - ubox.lo - w) * (newpos / thumb.max)
  2. vp.hi = vp.lo + w
  3. goto update
• resize or content change:
  1. newpos = (vp.lo - ubox.lo) / (ubox.hi - ubox.lo - w) * thumb.max
  2. if newpos != thumb.pos: update thumb

clamp_hi:

1. if vp.hi > ubox.hi
   1. vp.hi = ubox.hi
2. vp.lo = vp.hi - w
3. goto update

clamp_lo:

1. if vp.lo < ubox.lo
   1. vp.lo = ubox.lo
2. vp.hi = vp.lo + w
3. goto update

update:

1. if vp.lo != old:
   1. newpos = (vp.lo - ubox.lo) / (ubox.hi - ubox.lo - w) * thumb.max
   2. if newpos != thumb.pos: update thumb
2. redraw