perf: round separator drag pane heights to prevent permanent full repaints

[NemeZZiZZ]

What

Round the pane heights computed during a separator drag to integers.

Why

SeparatorWidget._pressedTouchMouseMoveEvent derives the drag distance from event.pageY (src/widget/SeparatorWidget.ts:116). On browsers that report sub-pixel mouse/touch coordinates (Firefox/Safari, or any browser under non-100% zoom / fractional HiDPI scaling), pageY is fractional, so the computed reducedPaneHeight / increasedPaneHeight are fractional and get stored via setBounding / setOptions.

A fractional pane height never equals container.clientHeight, which is always an integer. The guard in DrawWidget.updateImp (src/widget/DrawWidget.ts:75-80) therefore fails on every subsequent update and keeps escalating to UpdateLevel.Drawer: from that moment on, every crosshair mousemove triggers a full main + overlay repaint of both adjacent panes, permanently, until reload.

Measured on the dev UMD bundle with a programmatic scenario (chart 800x500, candle pane + VOL pane, 500 bars; 50 crosshair mousemoves; canvas repaints counted via clearRect instrumentation, fractional pageY drag simulated through the real event path):

50 crosshair moves	main canvas repaints	overlay repaints	pane heights after drag
before drag	0	125	373px / 100px
after fractional drag (before fix)	100	125	381.22px / 91.78px (stuck fractional)
after fractional drag (with fix)	0	125	381px / 92px (integer)

How

Two Math.round() calls at the source, where the heights are computed:

1. reducedPaneHeight = Math.round(Math.max(startDragReducedPaneHeight - Math.abs(dragDistance), reducedPaneMinHeight))
2. increasedPaneHeight = Math.round(startDragIncreasedPaneHeight + diffHeight)

This is safe because:

• The layout itself only produces integer pane heights (Chart._chartBounding is Math.floor'd), so rounding can only converge with the layout, never diverge from it.
• Rounding both panes independently can drift their sum by at most 1px; the flexible (candle) pane absorbs any remainder on the very next layout({ measureHeight: true }) call, which the drag handler already performs on every move.
• Integer heights make bounding.height === container.clientHeight hold again, so normal updates stay at UpdateLevel.Overlay.

Verification

• pnpm type-check — 0 errors
• pnpm code-lint (biome) — 0 errors
• Benchmark scenario above re-run on the patched bundle: main canvas repaints per 50 crosshair moves drop from 100 to 0 (identical to the pre-drag baseline), and pane heights are integers after the drag.

Notes

• Chrome currently reports integer pageY for mouse events, so the fractional coordinate path mainly affects Firefox/Safari and zoomed/HiDPI environments; the benchmark simulates a fractional pageY to exercise the same code path.
• A user can still set a fractional pane height explicitly through setPaneOptions; that API path is intentionally out of scope for this PR.