Leaflet map doesn't show in tab

The problem is that the Leaflet map doesn't show in tab when you click on the tab. It shows properly out of the tab but I want to display it in the tab. If you press in browser CTRL and + or CTRL and - (Zoom in and out) then the map shows up. The test file is attached so you can try it yourself straightaway. Click on the map tab to see what I'm talking about and then zoom in and out in browser and bingo ! The map is displayed. It should be 10 minute job for people who know how to sort this things out.