fix(coding-agent): bound compaction progress preview

packages/coding-agent/src/modes/interactive/changes.md

@@ -1,5 +1,28 @@
 # changes
 
+## bounded compaction progress preview (2026-07-24)
+
+### What changed
+
+- Streamed compaction summary progress is normalized and rendered as a single `TruncatedText` row beneath the active
+  compaction indicator.
+- Multiline summary content no longer expands the transient status container or pushes the editor and transcript
+  through repeated terminal viewport remaps.
+
+### Why
+
+- Compaction summaries can contain thousands of characters and many newlines. Rendering that temporary content
+  verbatim above the editor made the composer move with every progress update and pushed prior output into scrollback,
+  which looked like transcript deletion.
+
+### Why extension system couldn't handle this
+
+- Compaction progress events and the status/editor container layout are private interactive-mode rendering surfaces.
+
+### Expected merge conflict zones
+
+- MEDIUM: `interactive-mode.ts` compaction progress event rendering.
+
 ## per-section thinking duration headers (2026-07-22)
 
 ### What changed

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -3561,11 +3561,13 @@ export class InteractiveMode {
 					event.text !== undefined ? event.text : `${this.autoCompactionProgressText}${event.delta ?? ""}`;
 				if (!nextText) break;
 				this.autoCompactionProgressText = nextText;
-				const preview = nextText.length > 4_000 ? `...${nextText.slice(nextText.length - 4_000)}` : nextText;
+				const previewSource = nextText.length > 4_000 ? `...${nextText.slice(nextText.length - 4_000)}` : nextText;
+				const preview = previewSource.replace(/\s+/g, " ").trim();
+				if (!preview) break;
 				this.statusContainer.clear();
 				this.statusContainer.addChild(this.activeStatusIndicator);
 				this.statusContainer.addChild(new Spacer(1));
-				this.statusContainer.addChild(new Text(theme.fg("muted", preview), 1, 0));
+				this.statusContainer.addChild(new TruncatedText(theme.fg("muted", preview), 1, 0));
 				this.ui.requestRender();
 				break;
 			}

packages/coding-agent/test/interactive-mode-compaction.test.ts

@@ -83,12 +83,16 @@ describe("InteractiveMode compaction events", () => {
 		await handleEvent.call(fakeThis, {
 			type: "compaction_progress",
 			reason: "extension",
-			delta: "live summary chunk",
+			delta: "live summary\nchunk",
 		});
 
-		const rendered = stripAnsi(statusContainer.children.flatMap((child) => child.render(120)).join("\n"));
-		expect(rendered).toContain("Compacting context");
-		expect(rendered).toContain("live summary chunk");
+		const rendered = statusContainer.children
+			.flatMap((child) => child.render(120))
+			.map((line) => stripAnsi(line))
+			.filter((line) => line.trim().length > 0);
+		expect(rendered).toHaveLength(2);
+		expect(rendered[0]).toContain("Compacting context");
+		expect(rendered[1]).toContain("live summary chunk");
 
 		fakeThis.autoCompactionLoader?.stop();
 	});