The problem seems to be: When marking cells or text with the left mouse key we need to capture the mouse pointer, else we wouldn't be informed about its whereabouts, if the mouse pointer leaves the worksheet.
If that worksheet now pops up a menu that causes the worksheet to (for the application unexpectedly) immediately loose the ownership on the mouse pointer causing an assert.
The problem seems to be: When marking cells or text with the left mouse key we need to capture the mouse pointer, else we wouldn't be informed about its whereabouts, if the mouse pointer leaves the worksheet. If that worksheet now pops up a menu that causes the worksheet to (for the application unexpectedly) immediately loose the ownership on the mouse pointer causing an assert.