The following changes solves this problem.
<pre><code class="diff">
diff --git a/app/views/issues/_edit.html.erb b/app/views/issues/_edit.html.erb
index 3afaee4ca9..1282c8113c 100644
--- a/app/views/issues/_edit.html.erb
+++ b/app/views/issues/_edit.html.erb
@@ -28,7 +28,7 @@
</fieldset>
<% end %>
<% if @issue.notes_addable? %>
- <fieldset><legend><%= l(:field_notes) %></legend>
+ <fieldset id='new-note'><legend><%= l(:field_notes) %></legend>
<%= f.text_area :notes, :cols => 60, :rows => 10, :class => 'wiki-edit', :no_label => true %>
<%= wikitoolbar_for 'issue_notes', preview_issue_path(:project_id => @project, :issue_id => @issue) %>

diff --git a/public/stylesheets/application.css b/public/stylesheets/application.css
index 7dde120079..ad6383a427 100644
--- a/public/stylesheets/application.css
+++ b/public/stylesheets/application.css
@@ -536,6 +536,8 @@ div.journal h4 img.gravatar {margin-left:-32px;}

#history div:target h4 {background-color:#DDEEFF;}

+#update fieldset#new-note { min-inline-size: auto; }
+
div#activity dl, #search-results { margin-left: 2em; }
div#activity dd, #search-results dd { margin-bottom: 1em; padding-left: 18px; font-size: 0.9em; }
div#activity dt.me .time { border-bottom: 1px solid #999; }

</code></pre>
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Marius, could you review this fix?
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Go MAEDA wrote:
> Marius, could you review this fix?

Looking right now, maybe we can include the fix for this issue in the next release (version:"4.0.3").
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Marius BALTEANU wrote:
> Go MAEDA wrote:
> > Marius, could you review this fix?
>
> Looking right now, maybe we can include the fix for this issue in the next release (version:"4.0.3").

Thanks. Setting the target version to 4.0.3.
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The proposed patch is not enough, it only fixes the issue in the notes block.

The description is also affected, please the below screenshot:
!description.png!

The old fix before Edit/Preview tabs was the rule @'min-width': inherit;@ from below:
<pre><code class="css">
fieldset.preview {
margin-top: 1em;
min-width: inherit;
background: url(../images/draft.png);
}
</code></pre>

My proposal is to backport the fix to all fieldset elements:
<pre><code class="diff">
vagrant@jessie:/vagrant/project/redmine$ git diff
diff --git a/public/stylesheets/application.css b/public/stylesheets/application.css
index 7dde120..15b32df 100644
--- a/public/stylesheets/application.css
+++ b/public/stylesheets/application.css
@@ -455,7 +455,7 @@ div.square {
form {display: inline;}
input, select {vertical-align: middle; margin-top: 1px; margin-bottom: 1px;}
input[type="submit"] { -webkit-appearance: button; }
-fieldset {border: 1px solid #e4e4e4; margin:0;}
+fieldset {border: 1px solid #e4e4e4; margin:0; min-width: inherit;}
legend {color: #333;}
hr { width: 100%; height: 1px; background: #ccc; border: 0;}
blockquote { font-style: italic; border-left: 3px solid #e0e0e0; padding-left: 0.6em; margin-left: 0;}
</code></pre>

I've tested in various pages in Chrome and I didn't see any issue, but any feedback is welcome.

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Marius's patch looks like a good option, it's committed. Thanks.
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relates,Closed,27758,Adds preview option to the wiki toolbar