fix(core): `resizeCanvas` should include preheader height, fixes #493 (#497)
fix(core): `resizeCanvas` should include preheader height, fixes #493 (#497)

* fix(core): `resizeCanvas` should include preheader height, fixes #493
- ref #493

* refactor(core): remove unnecessary preheader height from calculation
6pac/SlickGrid,6pac/SlickGrid