[GTK] Speed up Combo setItems/removeAll/remove for large item counts

[vogella]

Problem

Setting or removing a large number of combo items (>5000) is very slow on GTK
(see eclipse-platform#506 — ~2.2s to add 5000 items,
~4s to clear them).

The bottleneck is not the storage: GtkComboBox recomputes its popup/cell-view
layout on every single model change while the model is attached, so each
inserted/removed row triggers a full relayout — O(n²)/O(n)-per-row behaviour.
(This is a different bottleneck from the older wrap-width regression in bug
489640, which was already mitigated with delayed wrap.)

Fix

Use the standard GTK idiom of detaching the GtkListStore model during bulk
updates so the widget reacts only once:

1. g_object_ref the model, then gtk_combo_box_set_model(handle, 0) to detach.
2. Bulk-modify the store directly while detached.
3. Re-attach the model and g_object_unref.

Applied to all three bulk operations:

Method	Before	After
setItems(...)	per-item gtk_combo_box_text_insert	direct gtk_list_store_insert/set while detached
removeAll()	gtk_combo_box_text_remove_all (reacts per row)	gtk_list_store_clear while detached
remove(start, end)	loop of gtk_combo_box_text_remove	direct gtk_list_store_remove (end→start) while detached

Adds one native binding, gtk_combo_box_set_model, in GTK.java. Both GTK3 and
GTK4 use a GtkTreeModel, so the same path works for both. Existing behaviour
(selection clearing, RTL handling, delayed wrap) is preserved. The direct-insert
path uses column 0 for text, matching the cell-renderer attribute already
hardcoded in createHandle.

remove(int index) (single item) is intentionally left as-is — detaching for one
row isn't worthwhile.

Notes / testing

• A new native method means os.c must be regenerated and the native library
  rebuilt; this happens in the GitHub CI build (natives are rebuilt each run),
  so this change needs the Linux GTK3/GTK4 CI run to validate linking and
  behaviour. It could not be compiled/tested locally.
• Adding the performance label would let CI run the timing tests.

Fixes eclipse-platform#506

🤖 Generated with Claude Code

---

Generated by Claude Code

[gemini-code-assist]

Code Review

This pull request optimizes bulk item operations in Combo.java (such as remove, removeAll, and setItems) by temporarily detaching the underlying GtkTreeModel from the GtkComboBox, performing the modifications directly on the GtkListStore, and then re-attaching the model. This prevents the widget from updating on every single change, resolving performance issues. Feedback on these changes includes a critical bug fix to restore the active selection index, which is lost when the model is detached, and an optimization to reuse a single GtkTreeIter allocation during bulk insertions instead of allocating it per item.

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[gemini-code-assist]

When the model is detached from the GtkComboBox using GTK.gtk_combo_box_set_model(handle, 0), the active selection index is reset to -1 (none) by GTK. When the model is re-attached, the selection is not automatically restored, resulting in a loss of the user's selection if the active item was not in the removed range.

To fix this regression, we should calculate the adjusted active index before detaching the model, and restore it (while blocking the CHANGED signal to avoid redundant events) after re-attaching the model.

	int index = GTK.gtk_combo_box_get_active (handle);
	if (start <= index && index <= end) clearText();
	int newIndex = -1;
	if (index != -1 && !(start <= index && index <= end)) {
		newIndex = index > end ? index - (end - start + 1) : index;
	}

	long model = handle != 0 ? GTK.gtk_combo_box_get_model (handle) : 0;
	if (model != 0) {
		/*
		 * Bug 506: Removing a range of items one-by-one is slow because the
		 * GtkComboBox reacts to every single row deletion. Detach the model,
		 * remove the rows directly from the GtkListStore (from the end of the
		 * range downwards so indices stay stable), and re-attach it so the
		 * widget only updates once.
		 */
		OS.g_object_ref (model);
		gtk_combo_box_toggle_wrap (false);
		GTK.gtk_combo_box_set_model (handle, 0);
		long iter = OS.g_malloc (GTK.GtkTreeIter_sizeof ());
		for (int i = end; i >= start; i--) {
			if (GTK.gtk_tree_model_iter_nth_child (model, iter, 0, i)) {
				GTK.gtk_list_store_remove (model, iter);
			}
		}
		OS.g_free (iter);
		GTK.gtk_combo_box_set_model (handle, model);
		if (newIndex != -1) {
			OS.g_signal_handlers_block_matched (handle, OS.G_SIGNAL_MATCH_DATA, 0, 0, 0, 0, CHANGED);
			GTK.gtk_combo_box_set_active (handle, newIndex);
			OS.g_signal_handlers_unblock_matched (handle, OS.G_SIGNAL_MATCH_DATA, 0, 0, 0, 0, CHANGED);
		}
		OS.g_object_unref (model);
		gtk_combo_box_toggle_wrap (true);
	}

[gemini-code-assist]

To optimize bulk insertions, we can allocate the GtkTreeIter once outside the loop and reuse it, rather than allocating and freeing it for every single item inside gtk_list_store_insert. This significantly reduces native memory allocation overhead (g_malloc / g_free) when setting a large number of items.

		OS.g_object_ref (model);
		gtk_combo_box_toggle_wrap (false);
		GTK.gtk_combo_box_set_model (handle, 0);
		GTK.gtk_list_store_clear (model);
		long iter = OS.g_malloc (GTK.GtkTreeIter_sizeof ());
		for (int i = 0; i < items.length; i++) {
			gtk_list_store_insert (model, iter, items [i], i);
		}
		OS.g_free (iter);
		GTK.gtk_combo_box_set_model (handle, model);
		OS.g_object_unref (model);
		gtk_combo_box_toggle_wrap (true);

[gemini-code-assist]

Update the signature of the private helper method gtk_list_store_insert to accept the pre-allocated iter pointer, avoiding per-item allocation overhead.

private void gtk_list_store_insert (long model, long iter, String string, int index) {
	byte[] buffer = Converter.wcsToMbcs (string, true);
	GTK.gtk_list_store_insert (model, iter, index);
	GTK.gtk_list_store_set (model, iter, 0, buffer, -1);
}

[github-actions]

Test Results (linux)

   94 files  ±0     94 suites  ±0   15m 20s ⏱️ +58s
4 597 tests ±0  4 373 ✅ ±0  224 💤 ±0  0 ❌ ±0 
3 383 runs  ±0  3 308 ✅ ±0   75 💤 ±0  0 ❌ ±0 

Results for commit ab7962a. ± Comparison against base commit 7f9089e.

[vogella]

Superseded by eclipse-platform#3401 (clean single commit, rebased on current master, shorter description).