🐛 Fix folding/unfolding rows giving invalid redraw range

Fixes #429

lib/mixins/canvas-drawer.js

@@ -696,8 +696,8 @@ export default class CanvasDrawer extends Mixin {
             offscreenRow: range.offscreenRow
           })
         } else if (change.end < range.start || change.start > range.end) {
-          // The change is outside the range but didn't added
-          // or removed lines
+          // The change is outside the range but didn't add
+          // or remove lines
           newIntactRanges.push(range)
         } else {
           // The change is within the range, there's one intact range
@@ -719,6 +719,15 @@ export default class CanvasDrawer extends Mixin {
                 end: range.end + change.screenDelta,
                 offscreenRow: range.offscreenRow + change.end + 1 - range.start
               })
+            } else if (change.screenDelta !== 0) {
+              // Lines are added or removed in the display buffer, the intact
+              // range starts in the next line after the change end plus the
+              // screen delta
+              newIntactRanges.push({
+                start: change.end + change.screenDelta + 1,
+                end: range.end + change.screenDelta,
+                offscreenRow: range.offscreenRow + change.end + 1 - range.start
+              })
             } else {
               // No lines are added, the intact range starts on the line after
               // the change end