Fix link event listener leak

The bug here is that _clearCurrentLink was being called only on the
line(s) that changed, but _askForLink was then being triggered
regardless. This caused more onRenderedViewportChange listeners to be
registered and for the thread blocking to get exponentially worse.

Fixes xtermjs#4341

Author: Tyriar

src/browser/Linkifier2.ts

@@ -309,19 +309,27 @@ export class Linkifier2 extends Disposable implements ILinkifier2 {
         }
       });
 
-      // Add listener for rerendering
+      // Listen to viewport changes re-render the link under the cursor when the viewport line
+      // changes
       if (this._renderService) {
         this._linkCacheDisposables.push(this._renderService.onRenderedViewportChange(e => {
+          // Sanity check, this shouldn't happen in practice as this listener would be disposed
+          if (!this._currentLink) {
+            return;
+          }
           // When start is 0 a scroll most likely occurred, make sure links above the fold also get
           // cleared.
           const start = e.start === 0 ? 0 : e.start + 1 + this._bufferService.buffer.ydisp;
-          const oldEvent = this._currentLink ? this._lastMouseEvent : undefined;
-          this._clearCurrentLink(start, e.end + 1 + this._bufferService.buffer.ydisp);
-          if (oldEvent && this._element) {
-            // re-eval previously active link after changes
-            const position = this._positionFromMouseEvent(oldEvent, this._element, this._mouseService!);
-            if (position) {
-              this._askForLink(position, false);
+          const end = this._bufferService.buffer.ydisp + 1 + e.end;
+          // Only clear the link if the viewport change happened on this line
+          if (this._currentLink.link.range.start.y >= start && this._currentLink.link.range.end.y <= end) {
+            this._clearCurrentLink(start, end);
+            if (this._lastMouseEvent && this._element) {
+              // re-eval previously active link after changes
+              const position = this._positionFromMouseEvent(this._lastMouseEvent, this._element, this._mouseService!);
+              if (position) {
+                this._askForLink(position, false);
+              }
             }
           }
         }));