There is a very annoying behavior in InDesign - when using the Shift+Arrow keys to extend the selection, the selection grows in a wrong direction.
To reproduce:
1. Click and drag the text cursor to select text.
2. Shift + Arrow left to make the selection smaller.
Bug: the selection starts to grow/shrink AT THE BEGINNING OF SELECTION, not at the end of the selection.
Why is this a problem?
When selecting text, the cursor must be left where the mouse button is released. If selecting from left to right, the cursor must be left at the end of the selection. Therefore, Shift+Arrow key must grow/shrink the selection at the end. InDesign does in an unexpected way, it leaves the cursor at the beginning of the selection!
BTW the cursor movement after selecting text is a random mess:
- Shift + Left arrow - extends the selection to the left
- after this: Shift + Right arrow extends the selection to the right
- after this: Shift + Left arrow shrinks the selection on the right (... ?)

The correct behavior (see practically all editors on this Earth, those all work correctly):
- When releasing the cursor at the beginning of the selection (selecting from right to left), Shift + Arrow keys grow/shrink the selection at the beginning (on the left side).
- When releasing the cursor at the end of the selection (selecting from left to right), Shift + Arrow keys grow/shrink the selection at the end (on the right side).