I am working in editable grid (Appian 7.9) and observed that if I try to select

I have the same problem. As a workaround, I didn't use the selection option in the editable grid, but I've added a check box in the inline grid row and set the value of it to the row id. The definition of the grid row may looks like this
a!gridRowLayout(
id:ri!index,
contents: {
a!checkboxField(
label:"Select" & ri!index,
choiceLabels :{""},
choiceValues :{if(rule!APN_isBlank(ri!index),{0},ri!index)},
saveInto :{
                              ri!selectedIndex
}
),
a!textField(
label: "Second Column " & ri!index,
value:ri!items[ri!index].secondColumnValue,
readOnly: true
),
a!textField(
label: "Third Columns " & ri!index,
value: ri!items[ri!index].ThirdColumnVale,
readOnly: true,
align: "RIGHT"
)
}

I have the same problem. As a workaround, I didn't use the selection option in the editable grid, but I've added a check box in the inline grid row and set the value of it to the row id. The definition of the grid row may looks like this
a!gridRowLayout(
id:ri!index,
contents: {
a!checkboxField(
label:"Select" & ri!index,
choiceLabels :{""},
choiceValues :{if(rule!APN_isBlank(ri!index),{0},ri!index)},
saveInto :{
                              ri!selectedIndex
}
),
a!textField(
label: "Second Column " & ri!index,
value:ri!items[ri!index].secondColumnValue,
readOnly: true
),
a!textField(
label: "Third Columns " & ri!index,
value: ri!items[ri!index].ThirdColumnVale,
readOnly: true,
align: "RIGHT"
)
}