fix(tui): auto-clear empty rows when content shrinks (#1095)

- Add height change detection (analogous to existing width detection)
- Auto-detect when content shrinks below maxLinesRendered and trigger
  full re-render to clear leftover empty rows
- Only triggers when no overlays are active (overlays need padding)

Fixes empty rows appearing below footer when:
- Closing /tree or other selectors
- Clearing multi-line editor content
- Any component shrinking

Also adds regression tests for resize handling and content shrinkage.

packages/tui/src/tui.ts

@@ -199,6 +199,7 @@ export class TUI extends Container {
 	public terminal: Terminal;
 	private previousLines: string[] = [];
 	private previousWidth = 0;
+	private previousHeight = 0;
 	private focusedComponent: Component | null = null;
 
 	/** Global callback for debug key (Shift+Ctrl+D). Called before input is forwarded to focused component. */
@@ -396,6 +397,7 @@ export class TUI extends Container {
 		if (force) {
 			this.previousLines = [];
 			this.previousWidth = -1; // -1 triggers widthChanged, forcing a full clear
+			this.previousHeight = -1; // -1 triggers heightChanged, forcing a full clear
 			this.cursorRow = 0;
 			this.hardwareCursorRow = 0;
 			this.maxLinesRendered = 0;
@@ -825,8 +827,9 @@ export class TUI extends Container {
 
 		newLines = this.applyLineResets(newLines);
 
-		// Width changed - need full re-render
+		// Width or height changed - need full re-render
 		const widthChanged = this.previousWidth !== 0 && this.previousWidth !== width;
+		const heightChanged = this.previousHeight !== 0 && this.previousHeight !== height;
 
 		// Helper to clear scrollback and viewport and render all new lines
 		const fullRender = (clear: boolean): void => {
@@ -851,16 +854,24 @@ export class TUI extends Container {
 			this.positionHardwareCursor(cursorPos, newLines.length);
 			this.previousLines = newLines;
 			this.previousWidth = width;
+			this.previousHeight = height;
 		};
 
 		// First render - just output everything without clearing (assumes clean screen)
-		if (this.previousLines.length === 0 && !widthChanged) {
+		if (this.previousLines.length === 0 && !widthChanged && !heightChanged) {
 			fullRender(false);
 			return;
 		}
 
-		// Width changed - full re-render
+		// Width or height changed - full re-render
-		if (widthChanged) {
+		if (widthChanged || heightChanged) {
+			fullRender(true);
+			return;
+		}
+
+		// Content shrunk below the working area and no overlays - re-render to clear empty rows
+		// (overlays need the padding, so only do this when no overlays are active)
+		if (newLines.length < this.maxLinesRendered && this.overlayStack.length === 0) {
 			fullRender(true);
 			return;
 		}
@@ -930,6 +941,7 @@ export class TUI extends Container {
 			this.positionHardwareCursor(cursorPos, newLines.length);
 			this.previousLines = newLines;
 			this.previousWidth = width;
+			this.previousHeight = height;
 			this.previousViewportTop = Math.max(0, this.maxLinesRendered - height);
 			return;
 		}
@@ -1076,6 +1088,7 @@ export class TUI extends Container {
 
 		this.previousLines = newLines;
 		this.previousWidth = width;
+		this.previousHeight = height;
 	}
 
 	/**

packages/tui/test/tui-render.test.ts

@@ -22,6 +22,133 @@ function getCellItalic(terminal: VirtualTerminal, row: number, col: number): num
 	return cell.isItalic();
 }
 
+describe("TUI resize handling", () => {
+	it("triggers full re-render when terminal height changes", async () => {
+		const terminal = new VirtualTerminal(40, 10);
+		const tui = new TUI(terminal);
+		const component = new TestComponent();
+		tui.addChild(component);
+
+		component.lines = ["Line 0", "Line 1", "Line 2"];
+		tui.start();
+		await terminal.flush();
+
+		const initialRedraws = tui.fullRedraws;
+
+		// Resize height
+		terminal.resize(40, 15);
+		await terminal.flush();
+
+		// Should have triggered a full redraw
+		assert.ok(tui.fullRedraws > initialRedraws, "Height change should trigger full redraw");
+
+		const viewport = terminal.getViewport();
+		assert.ok(viewport[0]?.includes("Line 0"), "Content preserved after height change");
+
+		tui.stop();
+	});
+
+	it("triggers full re-render when terminal width changes", async () => {
+		const terminal = new VirtualTerminal(40, 10);
+		const tui = new TUI(terminal);
+		const component = new TestComponent();
+		tui.addChild(component);
+
+		component.lines = ["Line 0", "Line 1", "Line 2"];
+		tui.start();
+		await terminal.flush();
+
+		const initialRedraws = tui.fullRedraws;
+
+		// Resize width
+		terminal.resize(60, 10);
+		await terminal.flush();
+
+		// Should have triggered a full redraw
+		assert.ok(tui.fullRedraws > initialRedraws, "Width change should trigger full redraw");
+
+		tui.stop();
+	});
+});
+
+describe("TUI content shrinkage", () => {
+	it("clears empty rows when content shrinks significantly", async () => {
+		const terminal = new VirtualTerminal(40, 10);
+		const tui = new TUI(terminal);
+		const component = new TestComponent();
+		tui.addChild(component);
+
+		// Start with many lines
+		component.lines = ["Line 0", "Line 1", "Line 2", "Line 3", "Line 4", "Line 5"];
+		tui.start();
+		await terminal.flush();
+
+		const initialRedraws = tui.fullRedraws;
+
+		// Shrink to fewer lines
+		component.lines = ["Line 0", "Line 1"];
+		tui.requestRender();
+		await terminal.flush();
+
+		// Should have triggered a full redraw to clear empty rows
+		assert.ok(tui.fullRedraws > initialRedraws, "Content shrinkage should trigger full redraw");
+
+		const viewport = terminal.getViewport();
+		assert.ok(viewport[0]?.includes("Line 0"), "First line preserved");
+		assert.ok(viewport[1]?.includes("Line 1"), "Second line preserved");
+		// Lines below should be empty (cleared)
+		assert.strictEqual(viewport[2]?.trim(), "", "Line 2 should be cleared");
+		assert.strictEqual(viewport[3]?.trim(), "", "Line 3 should be cleared");
+
+		tui.stop();
+	});
+
+	it("handles shrink to single line", async () => {
+		const terminal = new VirtualTerminal(40, 10);
+		const tui = new TUI(terminal);
+		const component = new TestComponent();
+		tui.addChild(component);
+
+		component.lines = ["Line 0", "Line 1", "Line 2", "Line 3"];
+		tui.start();
+		await terminal.flush();
+
+		// Shrink to single line
+		component.lines = ["Only line"];
+		tui.requestRender();
+		await terminal.flush();
+
+		const viewport = terminal.getViewport();
+		assert.ok(viewport[0]?.includes("Only line"), "Single line rendered");
+		assert.strictEqual(viewport[1]?.trim(), "", "Line 1 should be cleared");
+
+		tui.stop();
+	});
+
+	it("handles shrink to empty", async () => {
+		const terminal = new VirtualTerminal(40, 10);
+		const tui = new TUI(terminal);
+		const component = new TestComponent();
+		tui.addChild(component);
+
+		component.lines = ["Line 0", "Line 1", "Line 2"];
+		tui.start();
+		await terminal.flush();
+
+		// Shrink to empty
+		component.lines = [];
+		tui.requestRender();
+		await terminal.flush();
+
+		const viewport = terminal.getViewport();
+		// All lines should be empty
+		assert.strictEqual(viewport[0]?.trim(), "", "Line 0 should be cleared");
+		assert.strictEqual(viewport[1]?.trim(), "", "Line 1 should be cleared");
+
+		tui.stop();
+	});
+});
+
 describe("TUI differential rendering", () => {
 	it("tracks cursor correctly when content shrinks with unchanged remaining lines", async () => {
 		const terminal = new VirtualTerminal(40, 10);