"Index or size negative or greater than allowed amount" error is displayed if the "Copy" or "Download" button is clicked for a zero-area selection

[SoftVision-CosminMuntean]

[Affected versions]:

• Nightly 61.0a1
• Beta 60.0b15

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a clean Firefox profile and the Screenshots Onboarding tour already visited.

[Steps to reproduce]:

1. Open Firefox browser and navigate to any website.
2. Click the "Page actions" menu and select the "Take a Screenshot" option.
3. Perform a selection that have zero-width or zero-height.
4. Click the "Copy" or "Download" button and observe the behavior.

[Expected result]:

• "Your selection is too small" error is displayed and the overlay is not dismissed.

[Actual result]:

• "Index or size negative or greater than allowed amount" error is displayed and the overlay is dismissed until you click again on the page.

[Notes]:

• We had a similar issue logged for for the "Save" button in #2957.

• On the latest Firefox 59.0.2 release a different message error is displayed: "BackgroundError: Error: Data is not a valid png image".

• Here is a screenshot wit browser console error:

• Here is a screen recording with the issue:

[chenba]

Sentry: https://sentry.prod.mozaws.net/operations/screenshots-addon-dev/issues/4287898/

Asa also reported this error message in the Screenshots Slack channel. But he was trying to take a shot of a tweet on Twitter, so I don't know if it's the same root cause.

[ghost]

Let's set the minimum size of X or Y >0px