Fix maths for scroll-triggered big frame parsing

Big frames (ie plus and minus 6 or so frame heights) should only be sent
when plus or minus 6 frame heights have been scrolled.

webext/src/dom/commands_mixin.js

@@ -204,7 +204,8 @@ export default (MixinBase) => class extends MixinBase {
   _mightSendBigFrames() {
     if (this._is_raw_text_mode) { return }
     const y_diff = this.dimensions.frame.y_last_big_frame - this.dimensions.frame.y_scroll;
-    const max_y_scroll_without_new_big_frame = 2 * this.dimensions.tty.height;
+    const max_y_scroll_without_new_big_frame =
+      (this.dimensions._big_sub_frame_factor - 1) * this.dimensions.tty.height;
     if (Math.abs(y_diff) > max_y_scroll_without_new_big_frame) {
       this.log(
         `Parsing big frames: ` +

webext/src/dom/dimensions.js

@@ -83,9 +83,9 @@ export default class extends utils.mixins(CommonMixin) {
   _calculateBigSubFrame() {
     this.frame.sub = {
       left: this.frame.x_scroll - (this._big_sub_frame_factor * this.tty.width),
-      top: this.frame.y_scroll - (this._big_sub_frame_factor * this.tty.height),
+      top: this.frame.y_scroll - (this._big_sub_frame_factor * this.tty.height * 2),
       width: this.tty.width + (this._big_sub_frame_factor * 2 * this.tty.width),
-      height: this.tty.height + (this._big_sub_frame_factor * 2 * this.tty.height),
+      height: this.tty.height + (this._big_sub_frame_factor * 2 * this.tty.height * 2),
     }
     this._limitSubFrameDimensions();
     this._scaleSubFrameToSubDOM();