If the chat window is too long, then its hard to see the calendar at the bottom of the chat. This makes it difficult to see the calendar and often miss the alerts given by calendar by making the font bold.
If your problem is the chat window being too large, move it to the right side by using the corresponding Labs feature "Right-side chat". Then, the calendar will always be visible on the left directly below the labels.
Enable "Navbar drag and drop" in Settings → Labs and then just drag and drop.