workspace: Highlight where dragged tab will be dropped (#34740)

Closes #18565

I could use some advice on the color palette / theming. A couple
options:

1. The `drop_target_background` color could be used for the border if we
didn't use it for the background of the tab. In VSCode, the background
color of tabs doesn't change as you're dragging, there's just a border
between tabs. My only concern with this option is that the current
`drop_target_background` color is a bit subtle when used for a small
area like a border.

2. Another option could be to add a `drop_target_border` theme color,
but I don't know how much complexity this adds to implementation
(presumably all existing themes would need to be updated?).

Demo:


https://github.com/user-attachments/assets/0b7c04ea-5ec5-4b45-adad-156dfbf552db

Release Notes:

- Highlight where a dragged tab will be dropped between two other tabs

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

crates/theme/src/default_colors.rs

@@ -54,6 +54,7 @@ impl ThemeColors {
             element_disabled: neutral().light_alpha().step_3(),
             element_selection_background: blue().light().step_3().alpha(0.25),
             drop_target_background: blue().light_alpha().step_2(),
+            drop_target_border: neutral().light().step_12(),
             ghost_element_background: system.transparent,
             ghost_element_hover: neutral().light_alpha().step_3(),
             ghost_element_active: neutral().light_alpha().step_4(),
@@ -179,6 +180,7 @@ impl ThemeColors {
             element_disabled: neutral().dark_alpha().step_3(),
             element_selection_background: blue().dark().step_3().alpha(0.25),
             drop_target_background: blue().dark_alpha().step_2(),
+            drop_target_border: neutral().dark().step_12(),
             ghost_element_background: system.transparent,
             ghost_element_hover: neutral().dark_alpha().step_4(),
             ghost_element_active: neutral().dark_alpha().step_5(),

crates/theme/src/fallback_themes.rs

@@ -115,6 +115,7 @@ pub(crate) fn zed_default_dark() -> Theme {
                 element_disabled: SystemColors::default().transparent,
                 element_selection_background: player.local().selection.alpha(0.25),
                 drop_target_background: hsla(220.0 / 360., 8.3 / 100., 21.4 / 100., 1.0),
+                drop_target_border: hsla(221. / 360., 11. / 100., 86. / 100., 1.0),
                 ghost_element_background: SystemColors::default().transparent,
                 ghost_element_hover: hover,
                 ghost_element_active: hsla(220.0 / 360., 11.8 / 100., 20.0 / 100., 1.0),

crates/theme/src/schema.rs

@@ -225,6 +225,10 @@ pub struct ThemeColorsContent {
     #[serde(rename = "drop_target.background")]
     pub drop_target_background: Option<String>,
 
+    /// Border Color. Used for the border that shows where a dragged element will be dropped.
+    #[serde(rename = "drop_target.border")]
+    pub drop_target_border: Option<String>,
+
     /// Used for the background of a ghost element that should have the same background as the surface it's on.
     ///
     /// Elements might include: Buttons, Inputs, Checkboxes, Radio Buttons...
@@ -747,6 +751,10 @@ impl ThemeColorsContent {
                 .drop_target_background
                 .as_ref()
                 .and_then(|color| try_parse_color(color).ok()),
+            drop_target_border: self
+                .drop_target_border
+                .as_ref()
+                .and_then(|color| try_parse_color(color).ok()),
             ghost_element_background: self
                 .ghost_element_background
                 .as_ref()

crates/theme/src/styles/colors.rs

@@ -59,6 +59,8 @@ pub struct ThemeColors {
     pub element_disabled: Hsla,
     /// Background Color. Used for the area that shows where a dragged element will be dropped.
     pub drop_target_background: Hsla,
+    /// Border Color. Used for the border that shows where a dragged element will be dropped.
+    pub drop_target_border: Hsla,
     /// Used for the background of a ghost element that should have the same background as the surface it's on.
     ///
     /// Elements might include: Buttons, Inputs, Checkboxes, Radio Buttons...
@@ -304,6 +306,7 @@ pub enum ThemeColorField {
     ElementSelected,
     ElementDisabled,
     DropTargetBackground,
+    DropTargetBorder,
     GhostElementBackground,
     GhostElementHover,
     GhostElementActive,
@@ -418,6 +421,7 @@ impl ThemeColors {
             ThemeColorField::ElementSelected => self.element_selected,
             ThemeColorField::ElementDisabled => self.element_disabled,
             ThemeColorField::DropTargetBackground => self.drop_target_background,
+            ThemeColorField::DropTargetBorder => self.drop_target_border,
             ThemeColorField::GhostElementBackground => self.ghost_element_background,
             ThemeColorField::GhostElementHover => self.ghost_element_hover,
             ThemeColorField::GhostElementActive => self.ghost_element_active,

crates/workspace/src/pane.rs

@@ -2478,8 +2478,19 @@ impl Pane {
                 },
                 |tab, _, _, cx| cx.new(|_| tab.clone()),
             )
-            .drag_over::<DraggedTab>(|tab, _, _, cx| {
-                tab.bg(cx.theme().colors().drop_target_background)
+            .drag_over::<DraggedTab>(move |tab, dragged_tab: &DraggedTab, _, cx| {
+                let mut styled_tab = tab
+                    .bg(cx.theme().colors().drop_target_background)
+                    .border_color(cx.theme().colors().drop_target_border)
+                    .border_0();
+
+                if ix < dragged_tab.ix {
+                    styled_tab = styled_tab.border_l_2();
+                } else if ix > dragged_tab.ix {
+                    styled_tab = styled_tab.border_r_2();
+                }
+
+                styled_tab
             })
             .drag_over::<DraggedSelection>(|tab, _, _, cx| {
                 tab.bg(cx.theme().colors().drop_target_background)