Add clarification to documentation for Collection-Repeat regarding dynamic item heights

[CoenWarmer]

The documentation for Collection Repeat should include clarification about the use of dynamic heights for items.

Currently the documentation states that a function can be used to set the height of an item. The documentation would be improved if it included an explicit mention that, while the dynamic setting of heights works, it is required that the heights of items are known before the collection is rendered.

In cases where the height of an item is dependent on the height of the content within the item, it isn't possible to calculate the height of an item through the DOM and feed that to the getItemHeight function. If it is possible, it apparently has performance consequences that negate the use of collection-repeat in the first place.

This should be clarified in the documentation.

This is also requested in issue #1691

[ionitron-bot]

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