I've dealt with a few cases where threadless headsets required a very thin shim-spacer on top of the "wedge washer" that fits into the top bearing cartridge.

This keeps the larger-diameter top cap of these headsets from contacting the upper cup or headtube, and also prevents any sealing rings from being pinched down with no clearance.

These are shims, perhaps a half-millimeter thick at most, with a radial thickness of perhaps 4mm, and fit snugly over the steer tube so having an ID of 1-1/8".

These shims are also handy for fine-tuning the spacing between the top of the headset and the top of the steer tube, so that no further cutting or filing of the steerer is needed. These shims never go directly under the top cap with the 6mm hole in it, but rather go between spacers, under the stem or under the headset's large-diameter top cap (the one that sometimes has a cone shape to act as an aesthetic reducer between the OD of the spacers and the OD of the upper headset and/or head tube).

Some headsets and/or complete framesets come with the thin spacer shims, allowing the installer to selection-fit the assembly for smooth steering, good sealing and minimal fine-tuning of the steerer length.
Sometimes I have a built back come back in during break-in and a shim needs to be added to resolve a binding headset where the large-OD upper cap of the headset is suddenly contacting the head tube (because the installed bearings and/or wedge-washer have settled).