Since Adobe Photoshop 4, they revamped the GUI and put new feature in working with multiple documents.

Tab in Adobe Photoshop 4

The tab feature is really nice, but it has some problem when dealing with very thin document when slicing in HTML authoring process.

Adobe Photoshop 4 tab goes wrong

The canvas window parent is often auto resized to an inconvenient size by Photoshop. The solution is hidden somewhere in the Window menu. Tick the "Application Frame" option and you’ll have tamed Adobe Photoshop 4! Very recommended to try.

Adobe Photoshop 4 Application Frame