It was already mentioned in QTBUG-113251 but looks like it was missed hence raising separate ticket.
Left clicking the [x] does nothing instead of closing the devtools:
 
 
| For Gerrit Dashboard: QTBUG-115976 | ||||||
|---|---|---|---|---|---|---|
| # | Subject | Branch | Project | Status | CR | V | 
| 495739,3 | Fix close button in devtools | dev | qt/qtwebengine | Status: MERGED | +2 | 0 | 
| 497481,2 | Fix close button in devtools | 6.6 | qt/qtwebengine | Status: MERGED | +2 | 0 | 
| 497482,3 | Fix close button in devtools | 6.5 | qt/qtwebengine | Status: MERGED | +2 | 0 |