[css-overflow] drop overflow columns and subsequent content with discard (w3c#12785)

See w3c#8721

Author: frivoal

css-overflow-4/Overview.bs

@@ -1059,8 +1059,6 @@ Fragmentation of Overflow: the 'continue' property</h3>
 			if it is not already.
 			[[!CSS-BREAK-3]]
 			Content after the first [=region break=] is not rendered (see below).
-			(If the box is a [=multi-column container=],
-			any [=overflow columns=] are also not rendered.)
 
 			Note: This [=region break=] might be <a lt="forced break">forced</a>
 			(e.g. imposed by 'max-lines' or by another mechanism,
@@ -1072,6 +1070,94 @@ Fragmentation of Overflow: the 'continue' property</h3>
 			(such as pagination of this box itself)
 			do not cause any content to be discarded.
 
+			Additionally,
+			if the box is a [=multi-column container=],
+			any [=overflow columns=] are also not rendered,
+			nor is any subsequent content--
+			including any [=out of flow=] content
+			whose static position is in or after overflow columns,
+			nor is any additional [=multicol line=] or [=multicol row=]
+			that would be generated to hold such content.
+
+			<div class=example id=discard-overflow-column>
+				In the absence of the 'continue' property,
+				based on [[css-multicol-1 inline]],
+				the following markup and associated CSS would be expected
+				to render as follows.
+
+				<xmp highlight=markup>
+					<div class="multicol">
+						<p>Column 1</p>
+						<p>Column 2</p>
+						<p>Column 3</p>
+						<p>Overflow column
+							<div class="spanner">Spanner 1</div>
+						</p>
+						<div class="spanner">Spanner 2</div>
+						<p>Following 1</p>
+						<p>Following 2</p>
+						<p>Following 3</p>
+						<div class="spanner">Spanner 3</div>
+						<p>Following 4</p>
+						<p>Following 5</p>
+						<p>Following 6</p>
+					</div>
+				</xmp>
+				<pre><code highlight=css>
+					.multicol {
+					  columns: 3;
+					  border: solid 1px;
+					}
+					.multicol p {
+					  break-after: column;
+					  break-inside: avoid;
+					}
+					.spanner {
+					  column-span: all;
+					  text-align: center;
+					}
+				</code></pre>
+				<!--
+					ommitted because incidental to the example, but needed for an exact rendering:
+					p { margin: 0; }
+					.multicol {
+					  width: 47ch;
+					  font-family: monospace;
+					  gap: 1ch;
+					}
+				-->
+				<figure>
+				<pre id=sample-rendering-overflow-multicolumn-1 style="width: 63ch">
+				<div style="border:solid 1px; width: 47ch;">Column 1        Column 2        Column 3        Overflow column
+				                   Spanner 1
+				                   Spanner 2
+				Following 1     Following 2     Following 3
+				                   Spanner 3
+				Following 4     Following 5     Following 6
+				</div></pre>
+				</figure>
+
+				There is one overflow column,
+				a spanner whose static position is within that overflow column,
+				another spanner whose static position is after the overflow column,
+				inflow content after the spanners and the overflow column,
+				and another spanner whose static position is in this subsequent inflow content.
+
+				If we additionally apply the following style,
+				everything starting from and including the overflow column is discarded.
+
+				<pre><code highlight=css>
+					.multicol {
+					  continue: discard;
+					  block-ellipsis: auto;
+					}
+				</code></pre>
+				<figure>
+				<pre id=sample-rendering-overflow-multicolumn-2>
+				<div style="border:solid 1px; width: 47ch;">Column 1        Column 2        Column 3…</div></pre>
+				</figure>
+
+			</div>
 		<dt><dfn>collapse</dfn>
 		<dd>
 			If the box is a [=block container=], then it must