[tannevaled]

Problem

<h1 style="white-space:nowrap;width:10px">Short text here</h1>: scrollWidth returned 10. All of client* and scroll* returned the border box.

Spec read

• CSSOM View scrollWidth: return the width of the element's scrolling area (0 with no box; the root uses the viewport). clientWidth: 0 for no box or an inline box; otherwise the width of the padding edge, minus any scrollbar.
• CSS Overflow 3 §3.3, the scrollable overflow area, is the union of: the padding box; the line boxes it contains; the border boxes of the boxes it is the containing block for; and descendants' scrollable overflow, clipped when overflow is not visible.

Root cause

js/dom.go mapped clientWidth/Height and scrollWidth/Height to borderW/borderH, the border box.

Change

js.BoxMetrics is an optional Metrics extension (ClientSize, ScrollSize) the binder uses for client* and scroll*. Without it they fall back to the border box, as before. The engine's metrics implement it from the box tree:

• client = padding box, 0 for an inline or box-less element;
• scroll = union of the padding box, its line boxes and their items, and descendants' border boxes (descending only into non-clipping ones), measured from the padding box's top-left. An inline element reports its border box.

Each read goes through the forced synchronous layout from #293. Not modelled: the root/body viewport cases of the spec, and end-side alignment padding.

Tests

• TestScrollAndClientSize (engine, RenderHTML with JS): nowrap overflow; a 5px-border/2px-padding box with an overflowing 200×100 child → client 54, scroll 202×102; no overflow → padding box. Failed on main (false,10,64,64,64,34,56,26).
• TestSessionBoxMetrics (js): values come from BoxMetrics; unknown node → 0.

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