fix(dashboard): precise hover on the conversations-over-time chart

Hover was off by ~one letterbox width on wide layouts — users had to
drag the cursor far right of the last data point to see its tooltip.
Root cause: the SVG uses a fixed 760×240 viewBox with preserveAspect-
Ratio's default "xMidYMid meet", which centers the content and leaves
empty letterbox bars on either side when the container is wider than
the viewBox aspect. The old hover math mapped clientX linearly onto
the full SVG DOM rect, so every pixel of letterbox shifted the snap
further from the real data point.

Switched to SVGSVGElement.getScreenCTM().inverse() to map screen
coords back to viewBox units correctly regardless of letterboxing,
scaling, or future transforms. The tooltip's horizontal offset also
now comes from the same CTM (mapping the snapped data point's viewBox
x back to a pixel offset inside the wrapper div), so the tooltip and
crosshair line up exactly on top of each other.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Arnas Donauskas 2026-04-17 23:19:13 +03:00
parent eb8d0b3ac6
commit 2aa3c50878
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@ -106,8 +106,14 @@ function LineSvg({
maxY: number
ticks: number[]
}) {
const [hoverIdx, setHoverIdx] = useState<number | null>(null)
// Hover state: both the snapped index AND the tooltip's pixel
// offset inside the wrapper div. They're stored together so the
// tooltip positions against the chart's actual rendered pixels,
// not against a raw viewBox percentage. See the precision note on
// the onMove handler below.
const [hover, setHover] = useState<{ idx: number; tooltipLeftPx: number } | null>(null)
const svgRef = useRef<SVGSVGElement>(null)
const wrapRef = useRef<HTMLDivElement>(null)
const chartW = VB_W - PADDING.left - PADDING.right
const chartH = VB_H - PADDING.top - PADDING.bottom
@ -123,40 +129,67 @@ function LineSvg({
const incomingPath = data.map((p, i) => `${i === 0 ? 'M' : 'L'}${xFor(i)},${yFor(p.incoming)}`).join(' ')
const outgoingPath = data.map((p, i) => `${i === 0 ? 'M' : 'L'}${xFor(i)},${yFor(p.outgoing)}`).join(' ')
// Mouse-move: snap to nearest data-point index.
// Mouse-move: use the SVG's current screen-CTM to map clientX
// back to viewBox coordinates. The previous rect-based math
// assumed the viewBox filled the SVG DOM box linearly, but
// `preserveAspectRatio="xMidYMid meet"` (the SVG default)
// letterboxes the content horizontally when the container is
// wider than the viewBox aspect — so hover snapped hundreds of
// pixels off on wide layouts. CTM-inverse correctly accounts for
// letterboxing, scaling, and any future transform changes.
useEffect(() => {
const svg = svgRef.current
if (!svg) return
const wrap = wrapRef.current
if (!svg || !wrap) return
const onMove = (e: MouseEvent) => {
const rect = svg.getBoundingClientRect()
const xPct = (e.clientX - rect.left) / rect.width
const xPx = xPct * VB_W
if (xPx < PADDING.left - 8 || xPx > VB_W - PADDING.right + 8) {
setHoverIdx(null)
const ctm = svg.getScreenCTM()
if (!ctm) return
const pt = svg.createSVGPoint()
pt.x = e.clientX
pt.y = e.clientY
const local = pt.matrixTransform(ctm.inverse())
const xVb = local.x
if (xVb < PADDING.left - 8 || xVb > VB_W - PADDING.right + 8) {
setHover(null)
return
}
const local = xPx - PADDING.left
const idx = Math.round(stepX === 0 ? 0 : local / stepX)
setHoverIdx(Math.max(0, Math.min(data.length - 1, idx)))
const relative = xVb - PADDING.left
const idx = Math.max(
0,
Math.min(data.length - 1, Math.round(stepX === 0 ? 0 : relative / stepX)),
)
// Map the snapped data-point's viewBox x back to screen, then
// subtract the wrapper's left edge — that pixel offset is what
// the absolutely-positioned tooltip div consumes. `xFor` is
// inlined here so the effect deps stay stable (it's a closure
// that'd otherwise be a new reference every render).
const dataPointVbX = PADDING.left + idx * stepX
const dataPointPt = svg.createSVGPoint()
dataPointPt.x = dataPointVbX
dataPointPt.y = 0
const screen = dataPointPt.matrixTransform(ctm)
const wrapRect = wrap.getBoundingClientRect()
setHover({ idx, tooltipLeftPx: screen.x - wrapRect.left })
}
const onLeave = () => setHoverIdx(null)
const onLeave = () => setHover(null)
svg.addEventListener('mousemove', onMove)
svg.addEventListener('mouseleave', onLeave)
return () => {
svg.removeEventListener('mousemove', onMove)
svg.removeEventListener('mouseleave', onLeave)
}
// xFor + yFor close over stepX, so stepX covers them.
}, [data, stepX])
const hovered = hoverIdx !== null ? data[hoverIdx] : null
const hoverX = hoverIdx !== null ? xFor(hoverIdx) : 0
const hovered = hover !== null ? data[hover.idx] : null
const hoverX = hover !== null ? xFor(hover.idx) : 0
// X-axis label strategy: show ~6 evenly-spaced labels regardless
// of range so the axis never looks crowded.
const labelStride = Math.max(1, Math.ceil(data.length / 6))
return (
<div className="relative w-full">
<div ref={wrapRef} className="relative w-full">
<svg
ref={svgRef}
viewBox={`0 0 ${VB_W} ${VB_H}`}
@ -225,7 +258,7 @@ function LineSvg({
/>
{/* Hover crosshair */}
{hoverIdx !== null && (
{hover !== null && (
<g pointerEvents="none">
<line
x1={hoverX}
@ -235,17 +268,20 @@ function LineSvg({
stroke="rgb(71 85 105)"
strokeDasharray="3 3"
/>
<circle cx={hoverX} cy={yFor(data[hoverIdx].incoming)} r={3.5} fill="#3b82f6" />
<circle cx={hoverX} cy={yFor(data[hoverIdx].outgoing)} r={3.5} fill="#10b981" />
<circle cx={hoverX} cy={yFor(data[hover.idx].incoming)} r={3.5} fill="#3b82f6" />
<circle cx={hoverX} cy={yFor(data[hover.idx].outgoing)} r={3.5} fill="#10b981" />
</g>
)}
</svg>
{/* Tooltip — absolute-positioned div so we get crisp text, not SVG */}
{hovered && hoverIdx !== null && (
{/* Tooltip — absolute-positioned div so we get crisp text, not
SVG-rendered text. The left offset comes from the CTM-based
mapping so it lines up with the actual crosshair pixel, not a
letterboxed viewBox percentage. */}
{hovered && hover !== null && (
<div
className="pointer-events-none absolute top-0 z-10 -translate-x-1/2 rounded-md border border-slate-700 bg-slate-950 px-2.5 py-1.5 text-[11px] shadow-lg"
style={{ left: `${(hoverX / VB_W) * 100}%` }}
style={{ left: `${hover.tooltipLeftPx}px` }}
>
<div className="font-medium text-white">{longDayLabel(hovered.day)}</div>
<div className="mt-1 flex flex-col gap-0.5">