Add width and height to .jspsych-content for percentage-based plugin dimensions

[Copilot]

Problem

The .jspsych-content element lacked explicit width and height properties, preventing plugins from using percentage-based dimensions like height: 100% to fill the display element. When plugin authors tried to use these CSS properties, they would collapse to zero height/width because the parent element had no defined size.

This forced plugin developers to either:

• Avoid percentage-based layouts entirely
• Use viewport units like 100vh which don't respect the display element boundaries
• Add custom sizing logic in their plugins

Solution

Added width: 100% and height: 100% to the .jspsych-content CSS class in src/index.scss:

.jspsych-content {
  text-align: center;
  margin: auto; /* this is for overflowing content */
  width: 100%;
  height: 100%;
}

This minimal change allows .jspsych-content to fill its parent .jspsych-content-wrapper (which has flex: 1 1 100% and width: 100%), enabling child elements to properly calculate percentage-based dimensions.

Impact

Before this change:

<!-- Plugin content with height: 100% would have 0 height -->
<div class="jspsych-content">
  <div style="height: 100%">I have no height!</div>
</div>

After this change:

<!-- Plugin content with height: 100% now fills the display element -->
<div class="jspsych-content" style="width: 100%; height: 100%">
  <div style="height: 100%">I fill the display element!</div>
</div>

Backward Compatibility

This change is fully backward-compatible:

• The nested DOM structure remains unchanged
• Progress bar functionality is preserved
• All 360 existing tests pass
• No breaking changes to plugin API or behavior

Testing

• Added new tests in tests/core/display-element-sizing.test.ts to verify DOM structure with and without progress bar
• All existing tests continue to pass
• Manually verified with a demo showing percentage-based height working correctly

Closes #3285

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Simplify DOM and CSS for the display element?</issue_title>
<issue_description>Currently when an HTMLElement becomes jsPsych's display element, the following structure is created (assuming the display element is a div):

<div class="jspsych-display-element">
  <div class="jspsych-content-wrapper">
    <div class="jspsych-content">
       /* plugins dump stuff here */
    </div>
  </div>
</div>

I don't recall why we have this nested structure. My best guess is that it has to do with the progress bar, which would need to be displayed in the .jspsych-display-element but not in the content that the plugin controls.

The main issue with the current arrangement is that .jspsych-content doesn't have a width or height, so a plugin can't use a rule like height: 100% to take up the entire display. A plugin could use height: 100vh; but this would be bad practice since it wouldn't respect the boundaries of the display element in cases where that element doesn't take up the full viewport.

I'd like to find a CSS and DOM structure that fixes this issue, while preserving the functionality of the progress bar.
</issue_description>

<agent_instructions>Solve this without breaking changes if possible. Include a changeset in the pull request.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #3617

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[changeset-bot]

🦋 Changeset detected

Latest commit: 090a38b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
jspsych	Patch

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