The shift+drag warship selection rectangle was drawn on a second
offscreen canvas, blitted onto the main canvas2D context every frame
via world-coord transform. It's a screen-space rectangle though, so
none of that math was load-bearing.
Replace with a `<div>` positioned via inline left/top/width/height in
screen pixels. Same color tinting (player territoryColor lightened
0.2, dashed border at 0.85 alpha, fill at 0.06). pointer-events:none
so it doesn't intercept the drag.
Drops ~95 LOC of canvas2D drawing (renderSelectionBox, drawDashedLine,
selectionBoxCanvas/Ctx, the redraw() init, the renderLayer() blit).
One step closer to retiring the canvas2D map canvas — UILayer's
per-unit selection outlines are the last canvas2D draws on it.
evanpelle
d1651017ea