Keep at least one collection-tree row selected

Cmd-clicking (or Space on) the last selected row deselected it, leaving the
collection tree with no selection while its items stayed shown. Add an opt-in
requireSelection option to VirtualizedTable that refuses to toggle off the last
remaining row, and enable it for the collection tree.

chrome/content/zotero/collectionTree.jsx

@@ -487,7 +487,9 @@ var CollectionTree = class CollectionTree extends LibraryTree {
 				getRowString: this.getRowString.bind(this),
 				
 				multiSelect: this.props.multiSelect,
-				
+				// The collection tree must always have a selection
+				requireSelection: true,
+
 				onItemContextMenu: (...args) => this.props.onContextMenu && this.props.onContextMenu(...args),
 
 				onKeyDown: this.handleKeyDown,

chrome/content/zotero/components/virtualized-table.jsx

@@ -379,6 +379,10 @@ class VirtualizedTable extends React.Component {
 
 		multiSelect: false,
 
+		// When true, the last selected row can't be toggled off, so the selection
+		// never becomes empty through user action
+		requireSelection: false,
+
 		onSelectionChange: noop,
 
 		// The below are for arrow-key navigation
@@ -443,6 +447,8 @@ class VirtualizedTable extends React.Component {
 
 		multiSelect: PropTypes.bool,
 
+		requireSelection: PropTypes.bool,
+
 		onSelectionChange: PropTypes.func,
 
 		// The below are for arrow-key navigation
@@ -823,6 +829,11 @@ class VirtualizedTable extends React.Component {
 			this.selection.shiftSelect(index, toggleSelection, shouldDebounce);
 		}
 		else if (toggleSelection) {
+			// Don't allow toggling off the last selected row when a selection is required
+			if (this.props.requireSelection
+					&& this.selection.count === 1 && this.selection.isSelected(index)) {
+				return;
+			}
 			this.selection.toggleSelect(index, shouldDebounce);
 		}
 		else if (moveFocused) {

test/tests/collectionTreeTest.js

@@ -723,6 +723,40 @@ describe("Zotero.CollectionTree", function () {
 		});
 	});
 
+	describe("selection", function () {
+		// Toggle a row the way a Cmd-click/Space does (through the table's selection handler)
+		function toggleRow(index) {
+			cv.tree._onSelection(index, false, true);
+		}
+
+		it("should keep a selection when the last selected row is toggled off", async function () {
+			let c = await createDataObject('collection');
+			await cv.selectByID("C" + c.id);
+			let row = cv.getRowIndexByID("C" + c.id);
+			assert.equal(cv.selection.count, 1);
+
+			// Toggling off the only selected row would empty the selection
+			toggleRow(row);
+
+			assert.equal(cv.selection.count, 1);
+			assert.isTrue(cv.selection.isSelected(row));
+		});
+
+		it("should allow toggling off a row when others remain selected", async function () {
+			let c1 = await createDataObject('collection');
+			let c2 = await createDataObject('collection');
+			await cv.selectByID("C" + c1.id);
+			let r2 = cv.getRowIndexByID("C" + c2.id);
+			toggleRow(r2);
+			assert.equal(cv.selection.count, 2);
+
+			// Toggling one off leaves the other
+			toggleRow(r2);
+			assert.equal(cv.selection.count, 1);
+			assert.isTrue(cv.selection.isSelected(cv.getRowIndexByID("C" + c1.id)));
+		});
+	});
+
 	describe("#onDrop()", function () {
 		/**
 		 * Simulate a drag and drop