Bug 1582019 Part 2 - Disable splitting absolute positioned multicol containers. r=dholbert

The crashtest contains an absolute positioning <dialog> multicol
container. If it is fragmented, we end up having a very wrong frame
tree.

Differential Revision: https://phabricator.services.mozilla.com/D49210

--HG--
extra : moz-landing-system : lando

layout/generic/crashtests/1582019.html

@@ -0,0 +1,22 @@
+<style>
+.x {
+  font-family: serif;
+  max-width: 0vmin;
+  top: 1vh
+}
+* {
+  border-right: blue 1px solid;
+  column-count: 9;
+}
+#a {
+  column-span: all;
+}
+#b {
+  position: relative;
+</style>
+<h6 id="a">C</h6>
+<l>q
+p f
+<pre id="b" wrap>
+<dialog style="word-break:break-all" open class="x">Cp</pre>
+<q>6/N_s/"eve"

layout/generic/crashtests/crashtests.list

@@ -755,4 +755,5 @@ pref(layout.css.column-span.enabled,true) load 1572901.html
 pref(layout.css.column-span.enabled,true) load 1573216.html
 load 1574552.html
 pref(layout.css.column-span.enabled,true) load 1574993.html
+pref(layout.css.column-span.enabled,true) load 1582019.html
 load 1586470.html

layout/generic/nsAbsoluteContainingBlock.cpp

@@ -735,6 +735,9 @@ void nsAbsoluteContainingBlock::ReflowAbsoluteFrame(
       // blocks yet
       !aDelegatingFrame->IsInlineFrame() &&
 
+      // Bug 1588623: Support splitting absolute positioned multicol containers.
+      !aKidFrame->IsColumnSetWrapperFrame() &&
+
       // Don't split things below the fold. (Ideally we shouldn't *have*
       // anything totally below the fold, but we can't position frames
       // across next-in-flow breaks yet.