Watching from a window without disturbing birds
A comfortable indoor position can reveal normal behaviour, provided the window does not become a source of disturbance.

Move the chair back
The closest seat to the glass is not necessarily the best watching place. Sitting a little farther back can make your movements less obvious and give you a wider view of the garden. Try several positions before settling on one. A chair that lets you see the hedge base and a tree branch may reveal more than a position facing only the feeder.
Avoid sudden gestures when a bird is close. If it stops feeding and watches you each time you move, increase the distance or change your position. Glass does not make an observer invisible, especially when a bright room places a moving silhouette behind it.
Work with the view you already have
Keep binoculars and a notebook within reach so that finding them does not interrupt every sighting. Focus initially on an empty branch at the likely distance. When a bird appears, you will need only a small adjustment.
Reflections can complicate both viewing and photography. Switching off an unnecessary indoor light may help, as can changing your angle to the glass. Do not lean out of an upstairs window or remove a safe barrier for a clearer picture. A slightly softened image is preferable to an unsafe position.
If birds collide with or repeatedly challenge their reflections in the window, that needs a separate response using suitable external window treatments and current collision-prevention guidance. A comfortable watching spot should not depend on leaving a hazardous pane untreated.
Give a quiet period a purpose
On a morning with few birds, map the places you can see: open ground, dense cover, water and higher perches. Mark the areas hidden from view. This makes later observations more honest. No birds seen means none seen from that position during that period, not that the whole garden was empty.
Try a short watch at the same time on several days. Record weather and any disturbance, such as gardening or a delivery. You may discover that birds use a part of the garden while another area is busy.
Keep the window closed if opening it changes the birds' behaviour or introduces noise near a nest. The point of watching indoors is to let ordinary activity continue. There is no need to turn the windowsill into a stage or bring birds closer for the sake of the view.