Since .xsd is and XML itself you need to change the XML formatting settitngs.

In IDEA settings (File -> Settings... or Ctrl+Alt+S) go to Editor -> Code Style -> XML and in Wrap Attributes dropdown choose Do Not Wrap.

Answer from streetturtle on Stack Overflow
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JetBrains
intellij-support.jetbrains.com › hc › en-us › community › posts › 206809055-Is-there-a-format-XML-command
Is there a format XML command? – IDEs Support (IntelliJ Platform) | JetBrains
I'm using IDEA 15.0.1 Ultimate Thanks for any pointers., ... Right in the main menu: "Code => Reformat Code" or just Ctrl-Alt-L Or "Help => Find Action" (which is Ctrl-Shift-A), then type "format" ...
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JetBrains
jetbrains.com › help › idea › working-with-xml.html
XML | IntelliJ IDEA Documentation
IntelliJ IDEA brings powerful support for XML that includes structure validation, formatting (Ctrl+Alt+L) and indentation (Ctrl+Alt+I according to the XML code style, importing unbound namespaces, viewing code structure, unwrapping and removing ...
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JetBrains
intellij-support.jetbrains.com › hc › en-us › community › posts › 206846135-Problems-with-formatting-xml-files
Problems with formatting xml files – IDEs Support (IntelliJ Platform) | JetBrains
Do you know of anything that can make formatting attributes work correctly, but not indentation? I am on OS X with build 9684. Regards, Stig Kleppe-Jørgensen ... I tried it again with build 9720 with the same results... Regards, Stig ... Ok, build 9720. Settings / Code Style / General / Use same settings for all file types = Off. Settings / Code Style / General / XML / Indent = 4 Settings / Code Style / General / XML / Continuation indent = 8 Formatted the XML below.
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Etendo
docs.etendo.software › developer-guide › etendo-classic › getting-started › installation › intellij-code-formatting
Code Formatting Style in IntelliJ - Etendo Documentation
First, download the following IntelliJ formatting file. ... Click the gear icon → Import Schema → IntelliJ IDEA code style XML.
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Google
google.github.io › styleguide › intellij-java-google-style.xml
intellij-java-google-style.xml
xmlns:android · xmlns:.* · BY_NAME · *:id · http://schemas.android.com/apk/res/android · style · *:.*Style
Find elsewhere
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LabKey
labkey.org › Documentation › wiki-page.view
Use IntelliJ for XML File Editing: /Documentation
In IntelliJ, select File > Settings > Languages and Frameworks. Select Schemas and DTDs. In the top panel, you should see (or add) an entry for http://labkey.org/data/xml pointing to the tableinfo.xsd file in your enlistment.
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Lions e-District
e-district.org › userfiles › file › 37275898036.pdf pdf
Format xml in intellij idea - e-district.org
Config Codestyles Start IntelliJ in Preferences Go to "Editor" - "Code Style" - "Schema" and choose the style previously imported from the drop-down box, optionally applies the style to a project only Click on "Project Copy" and select "Use for" Project "project settings In idea> = 10, use files -> export / import settings. Eclipse Hippo-Eclipse- Formatting.xml Important: it is still necessary to configure to use the spaces instead of the tabs.
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JetBrains
jetbrains.com › help › rider › Code_Cleanup__Usage_Scenarios__Reformatting_XML.html
Reformat XML Doc comments | JetBrains Rider Documentation
In the profile preferences on the right, expand the Reformat code node, expand the node of the desired language (C#, Visual Basic .NET, or JavaScript/TypeScript) and select Reformat embedded XML doc comments. Click Save in the Settings dialog to apply the modifications and let JetBrains Rider choose where to save them, or save the modifications to a specific settings layer by choosing this layer from the Save selector.
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GitHub
github.com › google › styleguide › blob › gh-pages › intellij-java-google-style.xml
styleguide/intellij-java-google-style.xml at gh-pages · google/styleguide
<?xml version="1.0" encoding="UTF-8"?> <code_scheme name="GoogleStyle"> <option name="OTHER_INDENT_OPTIONS"> <value> <option name="INDENT_SIZE" value="2" /> <option name="CONTINUATION_INDENT_SIZE" value="4" /> <option name="TAB_SIZE" value="2" /> <option name="USE_TAB_CHARACTER" value="false" /> <option name="SMART_TABS" value="false" /> <option name="LABEL_INDENT_SIZE" value="0" /> <option name="LABEL_INDENT_ABSOLUTE" value="false" /> <option name="USE_RELATIVE_INDENTS" value="false" /> </value> </option> <option name="INSERT_INNER
Author   google
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JetBrains
jetbrains.com › help › idea › xml-refactorings.html
XML refactorings | IntelliJ IDEA Documentation
The XML refactoring support is ... and enabled by default. Place the caret at the element that you want to refactor and in the main menu, go to Refactor | XML Refactorings | <Refactoring_Name>....
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GitHub
github.com › duraspace › codestyle › blob › master › ide-support › intellij.md
codestyle/ide-support/intellij.md at master · duraspace/codestyle
IntelliJ IDEA allows you to bulk reformat entire projects or code modules ... Leave "Rearrange entries" unchecked (This will rearrange methods/variables based on arrangement settings in IDEA. This is not necessary.) Under Filters → Check File Masks and enter in "*.java, *.xml" (to ensure bulk reformatting only applies to Java and XML files)
Author   duraspace