Your input is ignored when you use the mouse to drag the vertical scroll bar handle of think-cell datasheets.
The problem was caused by a bug in Excel 2013. It could be reproduced without think-cell.
Reproduction without think-cell
Please perform the following steps:
Start Word 2013 or PowerPoint 2013 with a new empty file.
In Word/PowerPoint: INSERT
→ Create New
→ Microsoft Excel Worksheet.
In the embedded worksheet's edit mode, click
→ Freeze Panes.
Microsoft fixed the problem in KB2956145.