Dynamic grid row height

«By default, the grid will display rows at 25px. You can change this for each row individually to give each row a different height.»

Use reasonable fixed minimum row height
Always render (screen/minHeight) last rows
Find maximum possible viewport start (in units of item number + % of item)
Measure average height of last rows
avgHeight = max(minHeight, lastRowAvgHeight)
targetHeight = avgHeight*rowCount + headerHeight
Set sizer to targetHeight
scrollPos = targetHeight > offsetHeight ? min(1, scrollTop / (targetHeight — offsetHeight)) : 0
First visible item will be Math.floor(scrollPos*maxPossibleViewportStart)
Additional scroll offset will be itemOffset = scrollPos*maxPossibleViewportStart-Math.floor(scrollPos*maxPossibleViewportStart)*firstVisibleItemHeight
First (top) placeholder height will be scrollTop-firstVisibleItemHeight+itemOffset
Second (middle) placeholder height will be avgHeight*nodeCount — sum(heights of all rendered rows) — (first placeholder height)

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