Implement duration-based smooth scrolling

src/browser/Viewport.ts

@@ -13,6 +13,12 @@ import { IRenderDimensions } from 'browser/renderer/Types';
 
 const FALLBACK_SCROLL_BAR_WIDTH = 15;
 
+interface ISmoothScrollState {
+  startTime: number;
+  origin: number;
+  target: number;
+}
+
 /**
  * Represents the viewport of a terminal, the visible area within the larger buffer of output.
  * Logic for the virtual scroll bar is included in this object.
@@ -36,6 +42,11 @@ export class Viewport extends Disposable implements IViewport {
 
   private _refreshAnimationFrame: number | null = null;
   private _ignoreNextScrollEvent: boolean = false;
+  private _smoothScrollState: ISmoothScrollState = {
+    startTime: 0,
+    origin: -1,
+    target: -1
+  };
 
   constructor(
     private readonly _scrollLines: (amount: number) => void,
@@ -168,6 +179,37 @@ export class Viewport extends Disposable implements IViewport {
     this._scrollLines(diff);
   }
 
+  private _smoothScroll(): void {
+    // Check valid state
+    if (this._isDisposed || this._smoothScrollState.origin === -1 || this._smoothScrollState.target === -1) {
+      return;
+    }
+
+    // Calculate position complete
+    const percent = this._smoothScrollPercent();
+    this._viewportElement.scrollTop = this._smoothScrollState.origin + Math.round(percent * (this._smoothScrollState.target - this._smoothScrollState.origin));
+
+    // Continue or finish smooth scroll
+    if (percent < 1) {
+      window.requestAnimationFrame(() => this._smoothScroll());
+    } else {
+      this._clearSmoothScrollState();
+    }
+  }
+
+  private _smoothScrollPercent(): number {
+    if (!this._optionsService.rawOptions.smoothScrollDuration || !this._smoothScrollState.startTime) {
+      return 1;
+    }
+    return Math.max(Math.min((Date.now() - this._smoothScrollState.startTime) / this._optionsService.rawOptions.smoothScrollDuration, 1), 0);
+  }
+
+  private _clearSmoothScrollState(): void {
+    this._smoothScrollState.startTime = 0;
+    this._smoothScrollState.origin = -1;
+    this._smoothScrollState.target = -1;
+  }
+
   /**
    * Handles bubbling of scroll event in case the viewport has reached top or bottom
    * @param ev The scroll event.
@@ -196,7 +238,23 @@ export class Viewport extends Disposable implements IViewport {
     if (amount === 0) {
       return false;
     }
-    this._viewportElement.scrollTop += amount;
+    if (!this._optionsService.rawOptions.smoothScrollDuration) {
+      this._viewportElement.scrollTop += amount;
+    } else {
+      this._smoothScrollState.startTime = Date.now();
+      if (this._smoothScrollPercent() < 1) {
+        this._smoothScrollState.origin = this._viewportElement.scrollTop;
+        if (this._smoothScrollState.target === -1) {
+          this._smoothScrollState.target = this._viewportElement.scrollTop + amount;
+        } else {
+          this._smoothScrollState.target += amount;
+        }
+        this._smoothScrollState.target = Math.max(Math.min(this._smoothScrollState.target, this._viewportElement.scrollHeight), 0);
+        this._smoothScroll();
+      } else {
+        this._clearSmoothScrollState();
+      }
+    }
     return this._bubbleScroll(ev, amount);
   }
 

src/common/services/OptionsService.ts

@@ -28,6 +28,7 @@ export const DEFAULT_OPTIONS: Readonly<ITerminalOptions> = {
   scrollback: 1000,
   scrollSensitivity: 1,
   screenReaderMode: false,
+  smoothScrollDuration: 0,
   macOptionIsMeta: false,
   macOptionClickForcesSelection: false,
   minimumContrastRatio: 1,

src/common/services/Services.ts

@@ -232,6 +232,7 @@ export interface ITerminalOptions {
   screenReaderMode: boolean;
   scrollback: number;
   scrollSensitivity: number;
+  smoothScrollDuration: number;
   tabStopWidth: number;
   theme: ITheme;
   windowsMode: boolean;

typings/xterm-headless.d.ts

@@ -169,6 +169,12 @@ declare module 'xterm-headless' {
      */
     scrollSensitivity?: number;
 
+    /**
+     * The duration to smoothly scroll between the origin and the target in
+     * milliseconds. Set to 0 to disable smooth scrolling and scroll instantly.
+     */
+    smoothScrollDuration?: number;
+
     /**
      * The size of tab stops in the terminal.
      */

typings/xterm.d.ts

@@ -201,6 +201,12 @@ declare module 'xterm' {
      */
     scrollSensitivity?: number;
 
+    /**
+     * The duration to smoothly scroll between the origin and the target in
+     * milliseconds. Set to 0 to disable smooth scrolling and scroll instantly.
+     */
+    smoothScrollDuration?: number;
+
     /**
      * The size of tab stops in the terminal.
      */