Virtual table is slow with many items

[kenwenzel]

I encounter exactly this (closed) issue with RAP 3.29.0:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=396172

The problem is that Table.updateScrollBars() is called for each item that is disposed via Table.destroyItem():

After investigating

org.eclipse.rap/bundles/org.eclipse.rap.rwt/src/org/eclipse/swt/widgets/Table.java

Line 2546 in 70285aa

if( ( style & SWT.NO_SCROLL ) == 0 ) {

I've come up with the following workaround:

Field styleField;
try {
	styleField = Widget.class.getDeclaredField("style");
	styleField.setAccessible(true);
} catch (NoSuchFieldException e) {
	styleField = null;
}

var table = ((TableViewer) viewer).getTable();

int style = table.getStyle();
boolean resetStyle = false;
try {
	// this is a hack to fix performance issues with RAP
	// by prevent scrollbar updates for each destroyed item
	if (styleField != null && (style & SWT.NO_SCROLL) == 0) {
		try {
			styleField.set(table, table.getStyle() | SWT.NO_SCROLL);
			resetStyle = true;
		} catch (IllegalAccessException e) {
			// ignore
		}
	}
	table.setItemCount(children.length);
} finally {
	if (resetStyle) {
		try {
			styleField.set(table, style);
		} catch (IllegalAccessException e) {
			// ignore
		}
	}
}

Is there anything that can be done to prevent updating the scrollbar state in this situation for each item?

I would propose to simply add some kind of flag isUpdatingItemCount = true that can be checked in Table.updateScrollBars() additionally to the style flag SWT.NO_SCROLL.

[kenwenzel]

Should I prepare a PR that has the chance to be accepted?