> For the complete documentation index, see [llms.txt](https://docs.flatredball.com/gum/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.flatredball.com/gum/gum-tool/editor-tab.md). # Editor Tab ## Introduction The Editor tab provides a real-time display of your selected element. Editing can be performed in the editor tab by grabbing and moving objects, size handles, and the rotation handle. The editor tab can be used to visually select objects by clicking on them.
## Canvas Size The Editor tab displays the canvas size with a dotted rectangle. The canvas is used for positioning and sizing instances which are placed directly in a screen (with no direct parent). {% hint style="info" %} The Canvas Size dropdown has been added for the January 2026 version of Gum. {% endhint %} The canvas can be adjusted by selecting from the dropdown in the Editor tab.

Guides dropdown

### Project Default The Project Default guides uses the value as set through the Project Properties tab. For more information see the [Project Properties page](/gum/gum-tool/project-properties.md#canvas-width-height). ### Customizing Canvas Size Dropdown You can customize available canvas sizes by editing the .gumx file. At the time of this writing the customization must be done by hand. To do this: 1. Locate the .gumx file for your project 2. Open the .gumx file in a text editor 3. Search for the `CustomCanvasSizes` tag You can remove or add new sizes by making changes here. If the entire `CustomCanvasSizes` tag is deleted, Gum automatically re-creates the default set of tags the next time it loads the .gumx file. ## Font Scale The Font Scale UI allows changing the global font scale of the project. This Font Scale value multiplies the font values for all Text instances in a project. This can be used to test your project at different global font scales. This is a preview value and does not change your project. Similarly, this value is not automatically applied at runtime - it must be explicitly set in code.
Changing the global Font Scale value does the following to Gum elements: * Text objects have their Font Scale value multiplied by this value. Note that this does not generate new bitmap fonts * Objects which are sized according to the size of Text instances (such as the buttons in the screenshot above) are resized appropriately * Objects which use `Width Units` of [Absolute Multiplied by Font Scale](/gum/gum-tool/gum-elements/general-properties/width-units.md#absolute-multiplied-by-font-scale) or `Height Units` of [Absolute Multiplied by Font Scale](/gum/gum-tool/gum-elements/general-properties/height-units.md#absolute-multiplied-by-font-scale) are resized appropriately. ## Preview {% hint style="info" %} Available in October 2026, or now if building Gum from source. {% endhint %} The **Preview** button, the icon-only play triangle at the far right of the Editor tab toolbar, opens your selected screen or component in a separate window running the real runtime. Buttons click, list boxes scroll, and text boxes take input, the same as they would in your shipped game, without wiring up your own game project first. Preview needs a saved project and a selected screen or component. If either is missing, Gum reports why in the Output tab instead of opening a window. While Preview is open, selecting a different screen or component in the tool swaps what Preview shows. Selecting a state in the **States** tab puts the previewed element in that state, so you can check each state of a component with the real runtime. Saving changes in the tool updates the Preview window automatically, the same way [hot reload](/gum/code/debugging/hot-reload.md) works for a game you run yourself. Preview renders with the same sibling order the tool's canvas uses. If **Sort by batch** is selected in the **Performance** tab, Preview sorts by batch too, so what you see matches the tool. If your project has a component with the `TooltipBehavior`, Preview shows that component when you hover over a control that has a `ToolTip`, the same as a game loading the project does. See [Tooltip](/gum/code/controls/tooltip.md). Because Preview loads your project the same way a shipped game would, missing texture or font files show up as the same errors a player would see. If a font looks wrong, generate its `FontCache` in the tool first, the same way you would before running your own game. Texture changes need Preview restarted; only font changes are picked up live. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.flatredball.com/gum/gum-tool/editor-tab.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.