fix(tui): preserved tmux viewport on offscreen shrink

Skipped repainting tmux panes when an offscreen shrink leaves the visible tail unchanged, and added regression coverage for the no-content-bytes contract.

Fixes #2046

packages/tui/CHANGELOG.md

@@ -29,6 +29,8 @@
 
 ### Fixed
 
+- Fixed tmux offscreen-shrink frames to skip repainting when the visible tail is unchanged, avoiding intermittent blank/refresh flashes in pane terminals ([#2046](https://github.com/can1357/oh-my-pi/issues/2046)).
+
 - Fixed `Loader` text updates to skip identical messages and preserve the rendered `Text` cache instead of invalidating it every timer tick.
 
 - Fixed fullscreen overlay alt-frame rendering to reuse the current line-preparation path instead of calling removed fitting helpers.

packages/tui/src/tui.ts

@@ -2049,7 +2049,9 @@ export class TUI extends Container {
 			newLines.length < this.#previousLines.length &&
 			naturalViewportTop !== prevViewportTop
 		) {
-			return { kind: "viewportRepaint" };
+			return this.#bottomAnchoredViewportUnchanged(newLines, height)
+				? { kind: "deferredMutation" }
+				: { kind: "viewportRepaint" };
 		}
 
 		// Direct-input shrink can also move the natural viewport upward even when
@@ -2437,6 +2439,17 @@ export class TUI extends Container {
 		return { kind: "liveRegionPinned", appendFrom, appendTo, renderViewportTop };
 	}
 
+	#bottomAnchoredViewportUnchanged(newLines: string[], height: number): boolean {
+		const previousViewportTop = Math.max(0, this.#previousLines.length - height);
+		const newViewportTop = Math.max(0, newLines.length - height);
+		for (let row = 0; row < height; row++) {
+			if ((newLines[newViewportTop + row] ?? "") !== (this.#previousLines[previousViewportTop + row] ?? "")) {
+				return false;
+			}
+		}
+		return true;
+	}
+
 	#planDeferredTailRepaint(newLines: string[], prevViewportTop: number, height: number): RenderIntent {
 		const row = prevViewportTop + height - 1;
 		if (row < 0 || row >= this.#previousLines.length || newLines.length !== this.#previousLines.length) {

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

@@ -1469,6 +1469,40 @@ describe("TUI terminal-state regressions", () => {
 			});
 		});
 
+		it("tmux: offscreen shrink preserving the visible tail emits no repaint bytes", async () => {
+			await withEnvPatch({ TMUX: "1", STY: undefined, ZELLIJ: undefined }, async () => {
+				const term = new UnknownViewportTerminal(40, 4, 10_000);
+				const tui = new TUI(term);
+				const component = new MutableLinesComponent([
+					"old-0",
+					"remove-me",
+					"old-2",
+					"old-3",
+					"tail-0",
+					"tail-1",
+					"tail-2",
+					"tail-3",
+				]);
+				tui.addChild(component);
+
+				try {
+					tui.start();
+					await settle(term);
+					expect(visible(term)).toEqual(["tail-0", "tail-1", "tail-2", "tail-3"]);
+
+					const writes = captureWrites(term);
+					component.setLines(["old-0", "old-2", "old-3", "tail-0", "tail-1", "tail-2", "tail-3"]);
+					tui.requestRender();
+					await settle(term);
+
+					expect(visible(term)).toEqual(["tail-0", "tail-1", "tail-2", "tail-3"]);
+					expect(writes).toEqual([]);
+				} finally {
+					tui.stop();
+				}
+			});
+		});
+
 		// Root cause family: the dirty/replay machinery assumes native scrollback
 		// can be cleared and rebuilt, which is never true inside a multiplexer —
 		// tmux owns pane history, reflows it on resize itself, and a "replay" can