fix(terminal): prevent ghost lines when resizing terminal panel

Clear the prompt region before xterm.js reflow to prevent ghost lines
caused by reflowCursorLine:false (xterm default) which excludes the
cursor row from reflow. When widening, wrapped prompt lines cannot merge
back, leaving stale fragments visible.

The fix walks up through isWrapped lines to find the prompt start, erases
that region, then calls fitAddon.fit() inside the write() callback to
ensure the erase is applied before reflow. Readline's SIGWINCH redraw
then writes a clean prompt at the new width.

Also removes the ResizeObserver (redundant with WorkspaceManager events),
removes the PTY resize suppression mechanism, and increases the resize
debounce to 300ms to avoid intermediate SIGWINCH during drag-resizing.

Author: abose

src/extensionsIntegrated/Terminal/TerminalInstance.js

@@ -104,10 +104,6 @@ define(function (require, exports, module) {
         this.searchAddon = null;
         this.$container = null;
         this._resizeTimeout = null;
-        this._resizeObserver = null;
-        this._lastCols = 0;
-        this._lastRows = 0;
-        this._suppressPtyResize = false;
         this._disposed = false;
 
         // Bound event handlers for cleanup
@@ -157,12 +153,6 @@ define(function (require, exports, module) {
         // Fit to container
         this._fit();
 
-        // Use ResizeObserver for reliable resize detection
-        this._resizeObserver = new ResizeObserver(() => {
-            this.handleResize();
-        });
-        this._resizeObserver.observe(this.$container[0]);
-
         // Set up custom key handler to intercept editor shortcuts
         this.terminal.attachCustomKeyEventHandler(this._customKeyHandler.bind(this));
 
@@ -175,9 +165,9 @@ define(function (require, exports, module) {
             }
         });
 
-        // Wire resize: terminal -> PTY (suppressed during _fit to control timing)
+        // Wire resize: terminal -> PTY
         this.terminal.onResize(({cols, rows}) => {
-            if (this.isAlive && !this._suppressPtyResize) {
+            if (this.isAlive) {
                 this.nodeConnector.execPeer("resizeTerminal", {id: this.id, cols, rows}).catch((err) => {
                     console.error("Terminal: resize error:", err);
                 });
@@ -282,56 +272,70 @@ define(function (require, exports, module) {
 
     /**
      * Fit the terminal to its container.
-     * Suppresses the automatic PTY resize during fit() and clears the
-     * prompt area to avoid garbled output caused by readline redrawing
-     * the prompt on top of xterm's reflowed buffer.
-     * Historical output above the prompt is preserved.
+     *
+     * Before reflowing, the prompt area is cleared so that stale wrapped
+     * text does not survive the reflow as a "ghost" line.  xterm.js
+     * defaults to reflowCursorLine: false, meaning the cursor row is
+     * excluded from reflow.  When the terminal widens, a multi-line
+     * wrapped prompt cannot merge back into one line; the first part
+     * remains as a visible ghost.  By erasing the prompt region first
+     * (walking up through isWrapped lines), the reflow has nothing
+     * stale to preserve, and readline's SIGWINCH redraw writes a
+     * clean prompt at the new width.
      */
     TerminalInstance.prototype._fit = function () {
-        if (this.fitAddon && this.$container && this.$container.is(":visible")) {
-            try {
-                const dims = this.fitAddon.proposeDimensions();
-                if (!dims || (dims.cols === this._lastCols && dims.rows === this._lastRows)) {
-                    return;
-                }
+        if (!this.fitAddon || !this.$container || !this.$container.is(":visible")) {
+            return;
+        }
 
-                this._lastCols = dims.cols;
-                this._lastRows = dims.rows;
-
-                // Suppress automatic PTY resize from onResize handler
-                this._suppressPtyResize = true;
-                this.fitAddon.fit();
-                this._suppressPtyResize = false;
-
-                if (this.isAlive) {
-                    // After reflow, clear only the prompt line and below to
-                    // remove garbled content from the reflow/SIGWINCH conflict.
-                    // Use cursor position after reflow — the cursor sits at the
-                    // end of the (possibly garbled) prompt. Clearing from the
-                    // start of the cursor row preserves all output above.
-                    const cursorY = this.terminal.buffer.active.cursorY;
-                    this.terminal.write("\x1b[" + (cursorY + 1) + ";1H\x1b[J");
-
-                    this.nodeConnector.execPeer("resizeTerminal", {
-                        id: this.id, cols: dims.cols, rows: dims.rows
-                    }).catch((err) => {
-                        console.error("Terminal: resize error:", err);
-                    });
+        try {
+            // Clear the prompt region before reflow to prevent ghost lines.
+            // xterm.js write() is asynchronous, so we must wait for the
+            // clear to be processed before calling fit().
+            if (this.terminal && this.isAlive) {
+                const buf = this.terminal.buffer.active;
+                let promptStart = buf.cursorY;
+
+                // Walk upward through wrapped lines to find prompt start
+                while (promptStart > 0) {
+                    const line = buf.getLine(buf.baseY + promptStart);
+                    if (!line || !line.isWrapped) {
+                        break;
+                    }
+                    promptStart--;
                 }
-            } catch (e) {
-                // Container might not be visible yet
+
+                // Erase from prompt start to end of screen, then fit
+                // once the erase has been applied to the buffer.
+                this.terminal.write(
+                    "\x1b[" + (promptStart + 1) + ";1H\x1b[J",
+                    () => {
+                        try {
+                            this.fitAddon.fit();
+                        } catch (e) {
+                            // Container might not be visible yet
+                        }
+                    }
+                );
+                return;
             }
+
+            this.fitAddon.fit();
+        } catch (e) {
+            // Container might not be visible yet
         }
     };
 
     /**
-     * Handle container resize - debounced
+     * Handle container resize — debounced so that only the final size
+     * triggers a reflow + PTY resize. This avoids garbled prompts from
+     * intermediate SIGWINCH signals during continuous drag-resizing.
      */
     TerminalInstance.prototype.handleResize = function () {
         clearTimeout(this._resizeTimeout);
         this._resizeTimeout = setTimeout(() => {
             this._fit();
-        }, 150);
+        }, 300);
     };
 
     /**
@@ -402,12 +406,8 @@ define(function (require, exports, module) {
             this.isAlive = false;
         }
 
-        // Dispose resize observer and xterm
+        // Dispose xterm
         clearTimeout(this._resizeTimeout);
-        if (this._resizeObserver) {
-            this._resizeObserver.disconnect();
-            this._resizeObserver = null;
-        }
         if (this.terminal) {
             this.terminal.dispose();
             this.terminal = null;