bug#50798: 28.0.50; Tab line close button is off-center until it is highlighted with the mouse

[Juri Linkov]

>> I don't understand how coordinates are calculated,
>> so I'll try to create a minimal test case for debugging.
>
> Thanks.

It seems this test case exposes the problem.
The face 'test-highlight' just inherits from 'test-tab',
but hovering over the image shifts it upwards outside of the view.
The test uses the header-line to show that the problem is the same
for both the tab-line and the header-line.

(defface test
  '((t :height 2.0))
  "")

(defface test-tab
  '((default :inherit test)
    (t :box (:line-width 1 :style released-button)))
  "")

(defface test-highlight
  '((default :inherit test-tab))
  "")

(setq header-line-format
      (list (propertize " + "
                        'display '(image :type xpm
                                         :file "tabs/new.xpm"
                                         :margin (2 . 0)
                                         :ascent center)
                        'face 'test-tab
                        'mouse-face 'test-highlight)))