Reflow on resize using similar logic to conpty

This aligns reflowing much closer to how conpty does it. This was always
an issue but only became a big issue recently because conpty 1.22+ opts
to passthrough sequences rather than reprinting aggressively. This means
that the conpty buffer being in sync with the xterm.js buffer is more
important, otherwise the cursor will show up in a seemingly random
position.

The existing reflow appears to differ somewhat in conpty, like it seems
to reflow at the word level, not the character level like xterm.js, but
refining that closer if not worth the effort since conpty may end up
relying on the terminal's buffer in the future[1].

Fixes #5319
Fixes #3513
Related #4231
Related microsoft/vscode#241978

[1]: https://github.com/microsoft/terminal/blob/main/doc/specs/%2313000%20-%20In-process%20ConPTY.md

package.json

@@ -95,7 +95,7 @@
     "jsdom": "^18.0.1",
     "mocha": "^10.1.0",
     "mustache": "^4.2.0",
-    "node-pty": "1.1.0-beta19",
+    "node-pty": "^1.1.0-beta31",
     "nyc": "^15.1.0",
     "source-map-loader": "^3.0.0",
     "source-map-support": "^0.5.20",

src/common/buffer/Buffer.ts

@@ -320,7 +320,37 @@ export class Buffer implements IBuffer {
     if (toRemove.length > 0) {
       const newLayoutResult = reflowLargerCreateNewLayout(this.lines, toRemove);
       reflowLargerApplyNewLayout(this.lines, newLayoutResult.layout);
+
+      // For conpty, it has its own copy of the buffer _without scrollback_ internally. Its behavior
+      // when reflowing larger is to insert empty lines at the bottom of the buffer as when lines
+      // unwrap conpty's view cannot pull scrollback down, so it adds empty lines at the end.
+      let removedInViewport = 0;
+      const isWindowsMode = this._optionsService.rawOptions.windowsMode || this._optionsService.rawOptions.windowsPty.backend !== undefined || this._optionsService.rawOptions.windowsPty.buildNumber !== undefined;
+      if (isWindowsMode) {
+        for (let i = (toRemove.length / 2) - 1; i >= 0; i--) {
+          if (toRemove[i * 2 + 0] > this.ybase + removedInViewport) {
+            removedInViewport += toRemove[i * 2 + 1];
+          }
+        }
+      }
+
       this._reflowLargerAdjustViewport(newCols, newRows, newLayoutResult.countRemoved);
+
+      // Apply empty lines for any removed in viewport for conpty.
+      if (isWindowsMode) {
+        if (removedInViewport > 0) {
+          for (let i = 0; i < removedInViewport; i++) {
+            // Just add the new missing rows on Windows as conpty reprints the screen with it's
+            // view of the world. Once a line enters scrollback for conpty it remains there
+            this.lines.push(new BufferLine(newCols, this.getNullCell(DEFAULT_ATTR_DATA)));
+          }
+          if (this.ybase === this.ydisp) {
+            this.ydisp += removedInViewport;
+          }
+          this.ybase += removedInViewport;
+          this.y -= removedInViewport;
+        }
+      }
     }
   }
 
@@ -352,7 +382,7 @@ export class Buffer implements IBuffer {
     const nullCell = this.getNullCell(DEFAULT_ATTR_DATA);
     // Gather all BufferLines that need to be inserted into the Buffer here so that they can be
     // batched up and only committed once
-    const toInsert = [];
+    const toInsert: { start: number, newLines: IBufferLine[] }[] = [];
     let countToInsert = 0;
     // Go backwards as many lines may be trimmed and this will avoid considering them
     for (let y = this.lines.length - 1; y >= 0; y--) {
@@ -467,6 +497,20 @@ export class Buffer implements IBuffer {
       this.savedY = Math.min(this.savedY + linesToAdd, this.ybase + newRows - 1);
     }
 
+    // For conpty, it has its own copy of the buffer _without scrollback_ internally. Its behavior
+    // when reflowing smaller is to reflow all lines inside the viewport, and removing empty or
+    // whitespace only lines from the bottom, until non-whitespace is hit in order to prevent
+    // content from being pushed into the scrollback.
+    let addedInViewport = 0;
+    const isWindowsMode = this._optionsService.rawOptions.windowsMode || this._optionsService.rawOptions.windowsPty.backend !== undefined || this._optionsService.rawOptions.windowsPty.buildNumber !== undefined;
+    if (isWindowsMode) {
+      for (let i = toInsert.length - 1; i >= 0; i--) {
+        if (toInsert[i].start > this.ybase + addedInViewport) {
+          addedInViewport += toInsert[i].newLines.length;
+        }
+      }
+    }
+
     // Rearrange lines in the buffer if there are any insertions, this is done at the end rather
     // than earlier so that it's a single O(n) pass through the buffer, instead of O(n^2) from many
     // costly calls to CircularList.splice.
@@ -520,6 +564,35 @@ export class Buffer implements IBuffer {
         this.lines.onTrimEmitter.fire(amountToTrim);
       }
     }
+
+    // Apply empty lines to remove calculated earlier for conpty.
+    if (isWindowsMode) {
+      if (addedInViewport > 0) {
+        let emptyLinesAtBottom = 0;
+        for (let i = this.lines.length - 1; i >= this.ybase + this.y; i--) {
+          const line = this.lines.get(i) as BufferLine;
+          if (line.isWrapped || line.getTrimmedLength() > 0) {
+            break;
+          }
+          emptyLinesAtBottom++;
+        }
+        const emptyLinesToRemove = Math.min(addedInViewport, emptyLinesAtBottom);
+        if (emptyLinesToRemove > 0) {
+          for (let i = 0; i < emptyLinesToRemove; i++) {
+            this.lines.pop();
+          }
+          if (this.ybase === this.ydisp) {
+            this.ydisp -= emptyLinesToRemove;
+          }
+          this.ybase -= emptyLinesToRemove;
+          this.y += emptyLinesToRemove;
+          this.lines.onDeleteEmitter.fire({
+            index: this.lines.length - emptyLinesToRemove,
+            amount: emptyLinesToRemove
+          });
+        }
+      }
+    }
   }
 
   /**