Initially when loading Performance Analytics dashboards, widgets have a default size. When their data and graph is loaded, the widgets resize to show the results properly (graphs, lists ,etc). The widget resizing process does not always work correctly for all widgets. 

The issue appears to be happening randomly for different widgets. When refreshing just the widget, it does load properly.

ServiceNow was able to reproduce the issue randomly using the following steps:

1. Log in to an instance.
2. If the instance is domain separated, switch to a domain in which Performance Analytics data is displayed.
3. Navigate to Performance Analytics > Dashboards.
4. Load a dashboard that has several widgets with charts as visualization.
5. Switch to another dashboard that also contains several widgets with charts as visualization.
6. Switch between the two dashboards several times.
   Note that, eventually, one or more of the widgets on a dashboard do not resize properly based on their content.

1. Activate the Canvas plugin.
2. Create a new tab on the dashboard.
3. Copy the widgets from the previous ("legacy") tab to the new tab created in step 2.
4. On the new tab, drag and drop widgets into position and resize them in width and height.
5. When the new tab layout is complete, delete the old tab.