A quircky behavior has suddenly shown up in Excel. When I load it, either by opening an Excel file or by starting up the application with a blank worksheet, as soon as I left-click in any cell, my cursor is stuck in the mode of extending the selection, as if I were holding down the mouse button. Moving the cursor outside of the worksheet area to any of the menus just causes the selection region to expand by scrolling the worksheet area, so I cannot use any of the menus once I have made that first click. The keyboard at that point is also completely ineffective, except for Alt-Tab, which does properly swap me out of Excel, and Ctrl-Alt-Del, which does bring up the close program window. But if I ask the close program window to End Task, I get a message that Excel cannot be closed. The only thing I have been able to do is swap out of Excel, right-click on its button in the task bar, and close it from there.
I have had other problems with sticky cursors on this computer (a Dell Latitude notebook with touchpad), which seem to have been caused by a sticky shift key because I've always been able to unstick the cursor by hitting the shift key. But hitting the shift key or other keys, as well as clicking the mouse buttons, nothing seems to unstick the cursor in Excel now once I have clicked on any cell. When I swap out of Excel to Word, the cursor works just fine.
This behavior makes Excel completely useless except for viewing spreadsheets already created, so I hope someone has some idea what could be causing it and how to fix it.
Thank you for your help.