fix(terminal): prevent prompt from being erased on tab switch and creation

The ghost-line resize fix in _fit() unconditionally erased the prompt
region before reflowing. This caused two bugs:

1. Tab switching: show() called _fit() which erased the prompt, but
   dimensions hadn't changed so fitAddon.fit() was a no-op — no
   SIGWINCH fired and the shell never redrew the prompt.

2. New terminal (panel hidden): spawn() used proposeDimensions() to
   create the PTY at the actual container size while xterm was still
   at default 80×24. The later _fit() saw a dimension mismatch, erased
   the prompt, and resized xterm — but the PTY was already at that size
   so SIGWINCH was a no-op.

Fix: only erase the prompt region when dimensions are actually changing,
and call fitAddon.fit() before spawn() so xterm and PTY start at the
same size.

Author: abose

src/extensionsIntegrated/Terminal/TerminalInstance.js

@@ -289,35 +289,43 @@ define(function (require, exports, module) {
         }
 
         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().
+            // Only clear the prompt region when dimensions are actually
+            // changing — i.e. a real reflow will happen.  When dimensions
+            // are unchanged (e.g. tab switch, panel re-focus) clearing
+            // would erase the prompt without a subsequent SIGWINCH to
+            // make the shell redraw it.
             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;
+                const proposed = this.fitAddon.proposeDimensions();
+                const dimensionsChanged = proposed &&
+                    (proposed.cols !== this.terminal.cols || proposed.rows !== this.terminal.rows);
+
+                if (dimensionsChanged) {
+                    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--;
                     }
-                    promptStart--;
-                }
 
-                // 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
+                    // 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;
+                    );
+                    return;
+                }
             }
 
             this.fitAddon.fit();

src/extensionsIntegrated/Terminal/main.js

@@ -306,6 +306,13 @@ define(function (require, exports, module) {
             panel.show();
         }
 
+        // Fit the terminal now that the panel is visible so xterm
+        // has the correct dimensions before the PTY is spawned.
+        // Without this, xterm stays at default 80x24 while the PTY
+        // is created at the actual container size, causing a later
+        // _fit() to erase the prompt without a real resize/SIGWINCH.
+        try { instance.fitAddon.fit(); } catch (e) { /* not ready */ }
+
         // Spawn PTY process
         await instance.spawn();
     }