Remove CSS `has` selector and improve various styles (#26891)

Replace  #26850

Major changes:

1. Remove all `has` selectors, it is still not supported by firefox.
Actually there could be some more general and clearer approaches
2. Remove `two-toggle-buttons`, the `.ui.buttons` just works well
3. Rewrite the `.ui.buttons` border styles, see the screenshots
4. Remove the "fine-tuning" paddings from the the flex children, they
could layout themselves well.

![image](https://github.com/go-gitea/gitea/assets/2114189/a32ed6f3-60f7-43d5-9492-62c45d2397f6)

![image](https://github.com/go-gitea/gitea/assets/2114189/5cb173c5-c942-4237-8cb4-2697220b3f06)

![image](https://github.com/go-gitea/gitea/assets/2114189/8a1c12b3-a632-48ff-b1a7-a01a4417f821)

![image](https://github.com/go-gitea/gitea/assets/2114189/46bde1bd-9113-4231-965d-6ec9076f6a3b)

3 files changed, 29 insertions, 38 deletions

diff --git a/web_src/css/base.css b/web_src/css/base.css
index 9d7c12525e..cbd3336d26 100644
--- a/web_src/css/base.css
+++ b/web_src/css/base.css
@@ -485,8 +485,10 @@ a.label,
   border-color: var(--color-primary);
 }
 
-.ui.action.input .button {
+.ui.action.input .ui.ui.button {
   border-color: var(--color-input-border);
+  padding-top: 0; /* the ".action.input" is "flex + stretch", so let the buttons layout themselves */
+  padding-bottom: 0;
 }
 
 /* currently used for search bar dropdowns in repo search and explore code */
@@ -2015,25 +2017,19 @@ a.ui.basic.label:hover {
   border-color: var(--color-secondary);
 }
 
-.ui.attached.header .right .button {
-  padding: 8px 10px;
-  font-weight: var(--font-weight-normal);
-}
-
-/* reduce height of buttons with dropdown icon */
-.ui.attached.header .right .button:has(.dropdown.icon) {
-  padding-top: 7px;
-  padding-bottom: 7px;
-}
-
 /* fix misaligned right buttons on box headers */
-.ui.attached.header .right:not(.dropdown) {
+.ui.attached.header > .ui.right {
   position: absolute;
   right: 0.78571429rem;
   top: 0;
   bottom: 0;
   display: flex;
-  align-items: center;
+}
+
+.ui.attached.header > .ui.right > .button,
+.ui.attached.header > .ui.right > .dropdown > .button {
+  padding: 8px 10px;
+  font-weight: var(--font-weight-normal);
 }
 
 /* if a .top.attached.header is followed by a .segment, add some margin */
diff --git a/web_src/css/modules/button.css b/web_src/css/modules/button.css
index 373b815d5c..a6c3019b8d 100644
--- a/web_src/css/modules/button.css
+++ b/web_src/css/modules/button.css
@@ -1,5 +1,6 @@
 /* this contains override styles for buttons and related elements */
 
+/* these styles changed the Fomantic UI's rules, Fomantic UI expects only "basic" buttons have borders */
 .ui.button,
 .ui.button:focus {
   background: var(--color-button);
@@ -52,26 +53,29 @@ a.btn:hover {
   color: inherit;
 }
 
-/* other button styles */
+/* By default, Fomantic UI doesn't support "bordered" buttons group, but Gitea would like to use it.
+And the default buttons always have borders now (not the same as Fomantic UI's default buttons, see above).
+It needs some tricks to tweak the left/right borders with active state */
 
-.ui.buttons .button:first-child {
-  border-left: 1px solid var(--color-light-border);
+.ui.buttons .button {
+  border-right: none;
 }
 
-.ui.buttons .button:first-child:has(+ .button.active) {
-  border-right: none !important;
+.ui.buttons .button:first-child {
+  border-left: 1px solid var(--color-light-border);
 }
 
-.ui.buttons .button + .button:not(.active) {
-  border-left: none;
+.ui.buttons .button:last-child {
+  border-right: 1px solid var(--color-light-border);
 }
 
-.two-toggle-buttons .button:not(.active):first-of-type {
-  border-right: none;
+.ui.buttons .button.active {
+  border-left: 1px solid var(--color-light-border);
+  border-right: 1px solid var(--color-light-border);
 }
 
-.two-toggle-buttons .button.active:last-of-type {
-  border-left: 1px solid var(--color-light-border);
+.ui.buttons .button.active + .button {
+  border-left: none;
 }
 
 .ui.labeled.button.disabled > .button,
diff --git a/web_src/css/repo.css b/web_src/css/repo.css
index cbf9fac76b..242682593e 100644
--- a/web_src/css/repo.css
+++ b/web_src/css/repo.css
@@ -437,8 +437,8 @@
   justify-content: center;
 }
 
-.pdf-content:has(.pdf-fallback-button) {
-  height: 100px;
+.pdf-content .pdf-fallback-button {
+  margin: 50px auto;
 }
 
 .repository.file.list .non-diff-file-content .plain-text {
@@ -1054,11 +1054,6 @@
   margin: 0;
 }
 
-/* fix fomantic's border-radius via :first-child with hidden elements */
-.collapsible-comment-box:has(.gt-hidden) {
-  border-radius: var(--border-radius) !important;
-}
-
 .repository.view.issue .comment-list .code-comment .comment-header {
   background: transparent;
   border-bottom: 0 !important;
@@ -1637,14 +1632,10 @@
 }
 
 .repository .diff-box .header:not(.resolved-placeholder) .button {
-  padding: 8px 12px;
+  padding: 0 12px;
   flex: 0 0 auto;
   margin-right: 0;
-}
-
-.repository .diff-box .header:not(.resolved-placeholder) .button:has(svg) {
-  padding-top: 6px;
-  padding-bottom: 6px;
+  height: 30px;
 }
 
 .repository .diff-box .resolved-placeholder {