reward banner stackable when clicking on "Send a Tip" multiple times

[kjozwiak]

Description

When clicking on Send a Tip.. via the Rewards panel in the URL, you'll stack several banners on top of each other. We shouldn't be stacking the same banner.

Steps to Reproduce

1. launch brave (used 0.55.17 Chromium: 70.0.3538.67)
2. enable rewards via brave://rewards
3. once enabled, visit a random website and wait a few seconds
4. click on the BAT icon in the URL and click on Send a Tip..
5. once the tipping banner appears, click on the BAT icon in the URL and repeat 4-5 a few times

You'll notice that you can stack the same banner multiple times.

Actual result:

Expected result:

The tip banner shouldn't be stackable.

Reproduces how often:

100% reproducible using the above STR.

Brave version (brave://version info)

Brave	0.55.17 Chromium: 70.0.3538.67 (Official Build) (64-bit)
Revision	9ab0cfab84ded083718d3a4ff830726efd38869f-refs/branch-heads/3538@{#1002}
OS	Mac OS X

Reproducible on current release:

• Does it reproduce on brave-browser dev/beta builds?

Yes, reproducible under beta & dev.

• Does it reproduce on browser-laptop?

N/A

[Miyayes]

Reproduced in v1.31+, so still technically valid issue. Will get triaged in next meeting.

[Miyayes]

No longer an issue with Contribution Banner 3.0.