fix(tui): skip clipped rows during layout painting

packages/tui/CHANGELOG.md

@@ -8,7 +8,7 @@
 - Fixed phantom alternate-screen text selection from unmatched mouse events when changing terminal pane focus.
 - Fixed spaces in searchable settings queries changing the selected value instead of filtering multi-word labels.
 - Fixed alternate-screen Kitty images crossing vertical layout clip boundaries and overlapping sticky regions while scrolling.
-- Fixed alternate-screen redraws retransmitting Kitty image data, dropping adjacent row content when reusing placements, and rendering fixed-basis scroll content twice per frame.
+- Fixed alternate-screen redraws retransmitting Kitty image data, dropping adjacent row content when reusing placements, rendering fixed-basis scroll content twice per frame, and scanning clipped transcript rows while painting.
 
 ### Added
 

packages/tui/src/layout.ts

@@ -304,10 +304,10 @@ function paintScrollbar(box: LayoutBox, screen: string[], totalWidth: number): v
 function paintBox(box: LayoutBox, screen: string[], totalWidth: number): void {
 	if (box.lines) {
 		const offset = box.lineOffset ?? 0;
-		for (let localRow = 0; localRow < box.rect.height; localRow++) {
-			const row = box.rect.y + localRow;
-			if (row < box.clip.y || row >= box.clip.y + box.clip.height || row < 0 || row >= screen.length) continue;
-			const sourceLine = box.lines[offset + localRow];
+		const firstRow = Math.max(box.rect.y, box.clip.y, 0);
+		const lastRow = Math.min(box.rect.y + box.rect.height, box.clip.y + box.clip.height, screen.length);
+		for (let row = firstRow; row < lastRow; row++) {
+			const sourceLine = box.lines[offset + row - box.rect.y];
 			if (sourceLine === undefined) continue;
 			let line = sourceLine.replace(OSC133_ZONE_PREFIX, "");
 			const imageMetadata = getKittyImageMetadata(line);

packages/tui/test/layout.test.ts

@@ -48,6 +48,26 @@ describe("viewport layout", () => {
 		assert.strictEqual(renderCount, 1);
 	});
 
+	it("paints only clipped rows from very large scroll content", () => {
+		const lineCount = 1_000_000_000;
+		const lines: string[] = [];
+		lines.length = lineCount;
+		lines[lineCount - 4] = "before";
+		lines[lineCount - 3] = "visible 1";
+		lines[lineCount - 2] = "visible 2";
+		lines[lineCount - 1] = "visible 3";
+		const transcript = new ScrollView(
+			{
+				render: () => lines,
+				invalidate: () => {},
+			},
+			{ follow: "end" },
+		);
+
+		const frame = renderLayoutFrame(transcript, 10, 3, () => {});
+		assert.deepStrictEqual(visibleLines(frame.lines), ["visible 1", "visible 2", "visible 3"]);
+	});
+
 	it("shrinks entries to their minimum sizes", () => {
 		const frame = renderLayoutFrame(
 			new VStack([