Updating language around full screen

[mbgower]

Closes #4664 by removing the one normative occurrence of "on a full screen" in an SC and removing the defined term.

Replaces with a phrase using the existing defined term "viewport"

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[gundulaniemann]

The original glossary entry, which now according to discussion on 2025-10-10 is subject of deletion, points to the fact that one one hand, the author may rely on commonly used monitors (excluding monitors from the 1980s) and on the other hand should not rely on high end monitors.
If rewording the sentence implies loosing this information, I would not like to do this change.

[mbgower]

I don't understand how the updated definition (without a screen size note) alters the basic outcome of this exercise. Regardless of whether the monitor is 12" or 28", if the user maximizes a window they should not have to scroll sideways to read a line of text.
The text that was in the note of the definition is still in the Understanding document. Non-normative text in a definition is no more 'valid' than non-normative text in an understanding document, so no non-normative guidance has been removed.

If we are going to be concerned about scoping on this, there is language in the understanding document noting that this is not intended for small screens:

The horizontal scrolling requirement is not intended to apply to small-screen devices where long words may be displayed on a single line and require users to scroll horizontally. For the purposes of this requirement, authors should ensure that content meets this requirement on standard desktop/laptop displays with the browser window maximized.

I'm not sure we want to open up that can of worms. But if we do want to engage with the note, it may be an idea to understand exactly how we want it to be understood and conveyed.

This SC simply updates the normative wording to make it clearer, as per the original issue. If there is support to perpetuate the existing note, I'm not going to object.