Fix the composition element's position

src/Buffer.ts

@@ -52,6 +52,12 @@ export class Buffer implements IBuffer {
     return this._hasScrollback && this.lines.maxLength > this._terminal.rows;
   }
 
+  public get isCursorInViewport(): boolean {
+    const absoluteY = this.ybase + this.y;
+    const relativeY = absoluteY - this.ydisp;
+    return (relativeY >= 0 && relativeY < this._terminal.rows);
+  }
+
   /**
    * Gets the correct buffer length based on the rows provided, the terminal's
    * scrollback and whether this buffer is flagged to have scrollback or not.

src/CompositionHelper.ts

@@ -63,6 +63,7 @@ export class CompositionHelper {
    * @param {CompositionEvent} ev The event.
    */
   public compositionupdate(ev: CompositionEvent): void {
+  console.log('compositionupdate');
     this.compositionView.textContent = ev.data;
     this.updateCompositionElements();
     setTimeout(() => {
@@ -193,26 +194,26 @@ export class CompositionHelper {
     if (!this.isComposing) {
       return;
     }
-    const cursor = <HTMLElement>this.terminal.element.querySelector('.terminal-cursor');
-    if (cursor) {
-      // Take .xterm-rows offsetTop into account as well in case it's positioned absolutely within
-      // the .xterm element.
-      const xtermRows = <HTMLElement>this.terminal.element.querySelector('.xterm-rows');
-      const cursorTop = xtermRows.offsetTop + cursor.offsetTop;
 
-      this.compositionView.style.left = cursor.offsetLeft + 'px';
+    if (this.terminal.buffer.isCursorInViewport) {
+      const cellHeight = Math.ceil(this.terminal.charMeasure.height * this.terminal.options.lineHeight);
+      const cursorTop = this.terminal.buffer.y * cellHeight;
+      const cursorLeft = this.terminal.buffer.x * this.terminal.charMeasure.width;
+
+      this.compositionView.style.left = cursorLeft + 'px';
       this.compositionView.style.top = cursorTop + 'px';
-      this.compositionView.style.height = cursor.offsetHeight + 'px';
+      this.compositionView.style.height = cellHeight + 'px';
-      this.compositionView.style.lineHeight = cursor.offsetHeight + 'px';
+      this.compositionView.style.lineHeight = cellHeight + 'px';
       // Sync the textarea to the exact position of the composition view so the IME knows where the
       // text is.
       const compositionViewBounds = this.compositionView.getBoundingClientRect();
-      this.textarea.style.left = cursor.offsetLeft + 'px';
+      this.textarea.style.left = cursorLeft + 'px';
       this.textarea.style.top = cursorTop + 'px';
       this.textarea.style.width = compositionViewBounds.width + 'px';
       this.textarea.style.height = compositionViewBounds.height + 'px';
       this.textarea.style.lineHeight = compositionViewBounds.height + 'px';
     }
+
     if (!dontRecurse) {
       setTimeout(() => this.updateCompositionElements(true), 0);
     }

src/Interfaces.ts

@@ -145,6 +145,7 @@ export interface IBuffer {
   scrollTop: number;
   savedY: number;
   savedX: number;
+  isCursorInViewport: boolean;
   translateBufferLineToString(lineIndex: number, trimRight: boolean, startCol?: number, endCol?: number): string;
   nextStop(x?: number): number;
   prevStop(x?: number): number;

src/xterm.css

@@ -52,6 +52,11 @@
 .terminal .xterm-helpers {
     position: absolute;
     top: 0;
+    /**
+     * The z-index of the helpers must be higher than the canvases in order for
+     * IMEs to appear on top.
+     */
+    z-index: 10;
 }
 
 .terminal .xterm-helper-textarea {