setInteractiveRegions only recorded visibleDragRegion inside the darwin
branch, so on Windows and Linux it stayed null and dragging fell back to
clamping against the whole PET_WINDOW_WIDTH x PET_WINDOW_HEIGHT window. That
window is mostly transparent padding around a mascot anchored bottom center,
so the mascot stopped short of every display edge by the width of that
padding and could not be dragged into a corner. Record the region on all
platforms; the reposition-on-restore step stays darwin-only.
Normalizing the region also clamped against the nominal constants. Renderer
regions are measured against the live viewport, so clamp against the real
content box instead -- the same petWindowContentExtent the shape clamp
already uses -- and let the shape clamp reuse that one normalizer.
The Windows shape clamp used the nominal PET_WINDOW_HEIGHT constant. Renderer
regions are measured against the live viewport, so once the content area was
taller than that constant the mascot -- which sits flush with the viewport
bottom -- got sliced off from below, while the task badge kept its own
unclamped rect and stayed visible. Clamp the shape to the real content box.
Dragging also moved the window with setPosition, which Windows resolves as
getSize() + setBounds(); that DIP round trip grows the window a pixel at a
time on fractional display scaling, and the drag timer fires every 16ms.
Restate the recorded size on every tick so dragging stays size-neutral.
