Fix status bar hiding first line when shown

[nangtrongvuon]

This was caused by not accounting for the status bar's frame during line calculation.

Fixes #487

Review Checklist

• [x] I have responded to reviews and made changes where appropriate.
• [x] I have tested the code
• [x] I have updated comments / documentation related to the changes I made.
• [x] I have rebased my PR branch onto xi-mac/master.

[jansol]

Seems that mouse events need similar treatment now... Clicking to place the cursor seems off by almost the height of a line here.