fix(tui): reduce unnecessary full redraws for better performance

- Remove height change detection (only width changes trigger full redraw)
- Change clearOnShrink default to false (use PI_CLEAR_ON_SHRINK=1 to enable)
- Fix viewport check to use previousLines.length instead of maxLinesRendered
  (prevents false positive redraws when appending lines after content shrunk)
- Add clearOnShrink setting to /settings in coding-agent
- Remove line truncation in custom message component (always show full content)

packages/coding-agent/src/modes/interactive/components/custom-message.ts

@@ -90,14 +90,6 @@ export class CustomMessageComponent extends Container {
 				.join("\n");
 		}
 
-		// Limit lines when collapsed
-		if (!this._expanded) {
-			const lines = text.split("\n");
-			if (lines.length > 5) {
-				text = `${lines.slice(0, 5).join("\n")}\n...`;
-			}
-		}
-
 		this.box.addChild(
 			new Markdown(text, 0, 0, this.markdownTheme, {
 				color: (text: string) => theme.fg("customMessageText", text),

packages/coding-agent/src/modes/interactive/components/settings-selector.ts

@@ -40,6 +40,7 @@ export interface SettingsConfig {
 	editorPaddingX: number;
 	autocompleteMaxVisible: number;
 	quietStartup: boolean;
+	clearOnShrink: boolean;
 }
 
 export interface SettingsCallbacks {
@@ -60,6 +61,7 @@ export interface SettingsCallbacks {
 	onEditorPaddingXChange: (padding: number) => void;
 	onAutocompleteMaxVisibleChange: (maxVisible: number) => void;
 	onQuietStartupChange: (enabled: boolean) => void;
+	onClearOnShrinkChange: (enabled: boolean) => void;
 	onCancel: () => void;
 }
 
@@ -312,6 +314,16 @@ export class SettingsSelectorComponent extends Container {
 			values: ["3", "5", "7", "10", "15", "20"],
 		});
 
+		// Clear on shrink toggle (insert after autocomplete-max-visible)
+		const autocompleteIndex = items.findIndex((item) => item.id === "autocomplete-max-visible");
+		items.splice(autocompleteIndex + 1, 0, {
+			id: "clear-on-shrink",
+			label: "Clear on shrink",
+			description: "Clear empty rows when content shrinks (may cause flicker)",
+			currentValue: config.clearOnShrink ? "true" : "false",
+			values: ["true", "false"],
+		});
+
 		// Add borders
 		this.addChild(new DynamicBorder());
 
@@ -363,6 +375,9 @@ export class SettingsSelectorComponent extends Container {
 					case "autocomplete-max-visible":
 						callbacks.onAutocompleteMaxVisibleChange(parseInt(newValue, 10));
 						break;
+					case "clear-on-shrink":
+						callbacks.onClearOnShrinkChange(newValue === "true");
+						break;
 				}
 			},
 			callbacks.onCancel,

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

@@ -258,6 +258,7 @@ export class InteractiveMode {
 		this.session = session;
 		this.version = VERSION;
 		this.ui = new TUI(new ProcessTerminal(), this.settingsManager.getShowHardwareCursor());
+		this.ui.setClearOnShrink(this.settingsManager.getClearOnShrink());
 		this.headerContainer = new Container();
 		this.chatContainer = new Container();
 		this.pendingMessagesContainer = new Container();
@@ -2996,6 +2997,7 @@ export class InteractiveMode {
 					editorPaddingX: this.settingsManager.getEditorPaddingX(),
 					autocompleteMaxVisible: this.settingsManager.getAutocompleteMaxVisible(),
 					quietStartup: this.settingsManager.getQuietStartup(),
+					clearOnShrink: this.settingsManager.getClearOnShrink(),
 				},
 				{
 					onAutoCompactChange: (enabled) => {
@@ -3084,6 +3086,10 @@ export class InteractiveMode {
 							this.editor.setAutocompleteMaxVisible(maxVisible);
 						}
 					},
+					onClearOnShrinkChange: (enabled) => {
+						this.settingsManager.setClearOnShrink(enabled);
+						this.ui.setClearOnShrink(enabled);
+					},
 					onCancel: () => {
 						done();
 						this.ui.requestRender();