RFR: 8178297: TableView scrolls slightly when adding new elements
jpereda at openjdk.java.net
Mon Sep 7 17:58:44 UTC 2020
This PR fixes an issue that happens when adding new items to a `TableView` or any other control that uses
`VirtualFlow`, if the scroll position is not 0: the virtual flow is slightly shifted down.
For instance, let's say that a cell has a layoutY of -10.0. After adding a new item to the table, during the layout
pass the new value is initially modified to -9.999999999999996 (`VirtualFlow::adjustByPixelAmount`) , and then, after
calling `VirtualFlow::positionCell` (that uses `snapSizeY(position)`) the layoutY is modified to -9.5, causing an
undesired positive shift of 0.5 pixels.
`Region` has different snap methods: `snapSizeX/Y` are used to snap node dimension values, like width or height, by
ceiling to the nearest pixel (which explains the -9.5 value), while `snapSpaceX/Y` are used to snap position values
like insets, by rounding to the nearest pixel (this will give the expected -10.0).
Therefore, this PR modifies `VirtualFlow::positionCell` to use the `snapSpaceX/Y` methods, since these are being used
to set the layout of the cell and not its size.
Test: A unity test has been included. It simulates the case of adding new items to the virtual flow after an initial
scroll. It currently fails after adding a few items, and it passes with the changes of this PR.
- Add test to verify the cell position remains constant when new cells are added to the virtualFlow.
- Use correct snap method to set cell layout
Stats: 25 lines in 2 files changed: 23 ins; 0 del; 2 mod
Fetch: git fetch https://git.openjdk.java.net/jfx pull/297/head:pull/297
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