The trendline snap indicator was 66 pixels out, and so was every other overlay.
Lightweight Charts reports coordinates from the plot area's origin. The chart
element also contains the price scales, so enabling the left scale for the daily
labels moved the plot 66px into the element — and each overlay positioned with
left: against the element inherited that error twice over. The cursor's
element-x was read as a plot-x, resolving a bar about 66px right of the pointer;
the indicator was then drawn at that bar's plot-x interpreted as element-x,
landing 66px left of where the bar is painted. Neither near the cursor nor near
the bar, and scaling with zoom — two bars at 30m, a dozen at 1m — which is why
it read as random rather than as an offset.
Every diagnostic number agreed with itself the whole time, because dot_y,
expected_y and bar_low_y all come from the same API and shared the same wrong
origin. Instrumentation cannot see a systematic error in its own frame of
reference; what found this was comparing canvas.width to element.clientWidth and
getting 0.894.
Overlays now live in one container positioned over the plot canvas and inherit
plot coordinates untranslated — snap dot and label, comments, anchor handles,
preview line, tooltip, price tag, context menu — and eventPoint subtracts the
same offset so a pointer position and a chart coordinate mean the same thing.
The container follows the plot on resize.
Verified in page pixels rather than through the coordinate API: with the cursor
placed 30px below a known bar's low, the dot lands 30px above the cursor, on
that low, labelled 7789.00 L against a bar low of 7789.
Device pixel ratio, viewport size, resize desynchronisation and the chart
scrolling under the gesture were each measured and ruled out before this. The
e2e helper computed expected times from element-relative x, the same mistake in
the tests, and is corrected here.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Chris Amow
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