chore(css): remove unneeded CSS vendor prefixes (#37903)

Removes redundant/obsolete WebKit prefixes:

- `-webkit-mask-*` — duplicate the unprefixed `mask-*` siblings already
present; every supported browser handles unprefixed CSS Masking
longhands.
- `-webkit-overflow-scrolling: touch` — a no-op outside iOS Safari <13.

Browser floor (all support unprefixed `mask`): Chrome 120+, Safari
15.4+, Firefox 53+, and PaleMoon/Goanna (verified: unprefixed `mask`
longhands implemented unconditionally in UXP).

---
This PR was written with the help of Claude Opus 4.8

Co-authored-by: Claude (Opus 4.8) <noreply@anthropic.com>

web_src/css/base.css

@@ -350,9 +350,7 @@ a.label,
   width: 16px;
   height: 16px;
   mask-size: cover;
-  -webkit-mask-size: cover;
   mask-image: var(--select-arrows);
-  -webkit-mask-image: var(--select-arrows);
   background: currentcolor;
 }
 

web_src/css/modules/checkbox.css

@@ -35,13 +35,9 @@ input[type="checkbox"]::before {
   height: var(--checkbox-size);
   clip-path: inset(var(--checkbox-size) 0 0 0);
   mask-image: var(--checkbox-mask-checked);
-  -webkit-mask-image: var(--checkbox-mask-checked);
   mask-size: 75%;
-  -webkit-mask-size: 75%;
   mask-repeat: no-repeat;
-  -webkit-mask-repeat: no-repeat;
   mask-position: center;
-  -webkit-mask-position: center;
 }
 
 input[type="checkbox"]:checked,
@@ -67,9 +63,7 @@ input[type="radio"]:disabled:checked {
 input[type="checkbox"]:indeterminate::before {
   clip-path: inset(0);
   mask-image: var(--checkbox-mask-indeterminate);
-  -webkit-mask-image: var(--checkbox-mask-indeterminate);
   mask-size: 75%;
-  -webkit-mask-size: 75%;
 }
 
 .ui.checkbox {

web_src/css/modules/codeeditor.css

@@ -127,8 +127,6 @@
   background-color: currentcolor;
   mask-image: var(--octicon-x);
   mask-size: cover;
-  -webkit-mask-image: var(--octicon-x);
-  -webkit-mask-size: cover;
   border-radius: var(--border-radius);
 }
 

web_src/css/modules/dropdown.css

@@ -209,7 +209,6 @@ select.ui.dropdown {
   overflow-x: hidden;
   overflow-y: auto;
   backface-visibility: hidden;
-  -webkit-overflow-scrolling: touch;
   border-top-width: 0 !important;
   outline: none;
   margin: 0 -1px;
@@ -379,7 +378,6 @@ select.ui.dropdown {
   overflow-x: hidden;
   overflow-y: auto;
   backface-visibility: hidden;
-  -webkit-overflow-scrolling: touch;
 }
 
 @media only screen and (max-width: 767.98px) {
@@ -653,7 +651,6 @@ select.ui.dropdown {
   overflow-x: hidden;
   overflow-y: auto;
   backface-visibility: hidden;
-  -webkit-overflow-scrolling: touch;
   min-width: 100% !important;
   width: auto !important;
 }

web_src/css/review.css

@@ -26,9 +26,7 @@
   height: 14px;
   background-color: var(--color-yellow); /* TODO: maybe it needs a new kind of color, there is no suitable "warning" color in the current palette */
   mask-image: var(--octicon-alert-fill);
-  -webkit-mask-image: var(--octicon-alert-fill);
   mask-size: contain;
-  -webkit-mask-size: contain;
 }
 
 .repository .diff-file-box .code-diff td.lines-escape {

web_src/css/shared/settings.css

@@ -21,9 +21,7 @@
   width: 14px;
   height: 14px;
   mask-size: cover;
-  -webkit-mask-size: cover;
   mask-image: var(--octicon-chevron-right);
-  -webkit-mask-image: var(--octicon-chevron-right);
   background: currentcolor;
   border: 1px solid var(--color-body); /* workaround https://bugzilla.mozilla.org/show_bug.cgi?id=1671784 */
 }