option to toggle editor window on/off

[mcdon-ron]

Requesting a feature to toggle the editor window on/off. MarkPad would then act as a viewer for rendered markdown.

Avoid letting a user toggle both views off. If preview is off, and editor is toggled off, preview would then be toggled on.

Maybe save the toggle state of the views between runs of MarkPad. May help for a user that only uses one of the views.

[daminibhattacharya]

@mcdon-ron Can I try to fix this issue?