resizable, dock: Improve Panel adjust size logic on layout changed. (#1588)

This is a follow up to #1560 I noticed a couple small issues with
resizable panels and their use in docks after experimenting a lot:
- Sometimes the resizable group would take about ~500ms to update after
a resize. This was caused by the `StackPanel` not getting notified and
therefore not redrawn when the `ResizableState` was notified, as well as
the notify in `adjust_to_container_size` not actually firing properly.
The latter was fixed by defering the notify, though I'm still not quite
sure why that is necessary.
- In a couple of places, the total size all the panel missmatches the
container size, causing small glitches the first time a panel is
resized. Update the code in those places to properly adjust all the
panel sizes so they match the container size.

---------

Co-authored-by: Jason Lee <huacnlee@gmail.com>

crates/ui/src/dock/stack_panel.rs

@@ -56,10 +56,12 @@ impl StackPanel {
     pub fn new(axis: Axis, _: &mut Window, cx: &mut Context<Self>) -> Self {
         let state = cx.new(|_| ResizableState::default());
 
-        // Bubble up the resize event.
-        let _subscriptions = vec![cx.subscribe(&state, |_, _, _: &ResizablePanelEvent, cx| {
-            cx.emit(PanelEvent::LayoutChanged)
-        })];
+        let _subscriptions = vec![
+            // Bubble up the resize event.
+            cx.subscribe(&state, |_, _, _: &ResizablePanelEvent, cx| {
+                cx.emit(PanelEvent::LayoutChanged)
+            }),
+        ];
 
         Self {
             axis,
@@ -253,7 +255,7 @@ impl StackPanel {
             Some(size) => size,
             None => {
                 let state = self.state.read(cx);
-                (state.total_size() / (state.sizes().len() + 1) as f32).max(PANEL_MIN_SIZE)
+                (state.container_size() / (state.sizes().len() + 1) as f32).max(PANEL_MIN_SIZE)
             }
         };
 

crates/ui/src/resizable/mod.rs

@@ -1,6 +1,8 @@
 use std::ops::Range;
 
-use gpui::{px, Along, App, Axis, Bounds, Context, ElementId, EventEmitter, Pixels, Window};
+use gpui::{
+    px, Along, App, Axis, Bounds, Context, ElementId, EventEmitter, IsZero, Pixels, Window,
+};
 
 use crate::PixelsExt;
 
@@ -66,23 +68,56 @@ impl ResizableState {
             ..Default::default()
         };
 
+        let size = size.unwrap_or(PANEL_MIN_SIZE);
+
+        // We make sure that the size always sums up to the container size
+        // by reducing the size of all other panels first.
+        let container_size = self.container_size().max(px(1.));
+        let total_leftover_size = (container_size - size).max(px(1.));
+
+        for (i, panel) in self.panels.iter_mut().enumerate() {
+            let ratio = self.sizes[i] / container_size;
+            self.sizes[i] = total_leftover_size * ratio;
+            panel.size = Some(self.sizes[i]);
+        }
+
         if let Some(ix) = ix {
             self.panels.insert(ix, panel_state);
-            self.sizes.insert(ix, size.unwrap_or(PANEL_MIN_SIZE));
+            self.sizes.insert(ix, size);
         } else {
             self.panels.push(panel_state);
-            self.sizes.push(size.unwrap_or(PANEL_MIN_SIZE));
+            self.sizes.push(size);
         };
+
         cx.notify();
     }
 
-    pub(crate) fn sync_panels_count(&mut self, axis: Axis, panels_count: usize) {
+    pub(crate) fn sync_panels_count(
+        &mut self,
+        axis: Axis,
+        panels_count: usize,
+        cx: &mut Context<Self>,
+    ) {
+        let mut changed = self.axis != axis;
         self.axis = axis;
+
         if panels_count > self.panels.len() {
             let diff = panels_count - self.panels.len();
             self.panels
                 .extend(vec![ResizablePanelState::default(); diff]);
             self.sizes.extend(vec![PANEL_MIN_SIZE; diff]);
+            changed = true;
+        }
+
+        if panels_count < self.panels.len() {
+            self.panels.truncate(panels_count);
+            self.sizes.truncate(panels_count);
+            changed = true;
+        }
+
+        if changed {
+            // We need to make sure the total size is in line with the container size.
+            self.adjust_to_container_size(cx);
         }
     }
 
@@ -114,7 +149,7 @@ impl ResizableState {
                 self.resizing_panel_ix = Some(resizing_panel_ix - 1);
             }
         }
-        cx.notify();
+        self.adjust_to_container_size(cx);
     }
 
     pub(crate) fn replace_panel(
@@ -127,7 +162,7 @@ impl ResizableState {
 
         self.panels[panel_ix] = panel;
         self.sizes[panel_ix] = old_size;
-        cx.notify();
+        self.adjust_to_container_size(cx);
     }
 
     pub(crate) fn clear(&mut self) {
@@ -135,8 +170,9 @@ impl ResizableState {
         self.sizes.clear();
     }
 
-    pub(crate) fn total_size(&self) -> Pixels {
-        self.sizes.iter().map(|s| s.as_f32()).sum::<f32>().into()
+    #[inline]
+    pub(crate) fn container_size(&self) -> Pixels {
+        self.bounds.size.along(self.axis)
     }
 
     pub(crate) fn done_resizing(&mut self, cx: &mut Context<Self>) {
@@ -168,7 +204,7 @@ impl ResizableState {
         if ix >= old_sizes.len() - 1 {
             return;
         }
-        let container_size = self.bounds.size.along(self.axis);
+        let container_size = self.container_size();
         self.sync_real_panel_sizes(cx);
 
         let move_changed = size - old_sizes[ix];
@@ -226,21 +262,25 @@ impl ResizableState {
         cx.notify();
     }
 
+    /// Adjust panel sizes according to the container size.
+    ///
     /// When the container size changes, the panels should take up the same percentage as they did before.
     fn adjust_to_container_size(&mut self, cx: &mut Context<Self>) {
-        // At least 1px so we don't divide by zero.
-        let container_size = self.bounds.size.along(self.axis).max(px(1.));
-        let total_size = self.total_size();
+        if self.container_size().is_zero() {
+            return;
+        }
+
+        let container_size = self.container_size();
+        let total_size = px(self.sizes.iter().map(|s| s.as_f32()).sum::<f32>());
 
         for i in 0..self.panels.len() {
             let size = self.sizes[i];
-            let fractional_size = size / total_size;
-            let new_size = container_size * fractional_size;
+            let ratio = size / total_size;
+            let new_size = container_size * ratio;
 
             self.sizes[i] = new_size;
             self.panels[i].size = Some(new_size);
         }
-
         cx.notify();
     }
 }

crates/ui/src/resizable/panel.rs

@@ -136,8 +136,8 @@ impl RenderOnce for ResizablePanelGroup {
 
         // Sync panels to the state
         let panels_count = self.children.len();
-        state.update(cx, |state, _| {
-            state.sync_panels_count(self.axis, panels_count);
+        state.update(cx, |state, cx| {
+            state.sync_panels_count(self.axis, panels_count, cx);
         });
 
         container