layout: Focus prev column when removing just created column

This works similar to tabs in Firefox: when you create a tab and then
close it without focusing any other tabs, the focus returns to the
previously-focused tab, rather than the usual next tab. The logic is
that in this case, the new tab was likely transient, and the user wants
to get right back to what they were doing.

In niri's case, this also serves as a workaround for wl-clipboard
spawning and destroying a window, messing up the focus.

src/layout.rs

@@ -145,6 +145,16 @@ pub struct Workspace<W: LayoutElement> {
 
     /// Animation of the view offset, if one is currently ongoing.
     view_offset_anim: Option<Animation>,
+
+    /// Whether to activate the previous, rather than the next, column upon column removal.
+    ///
+    /// When a new column is created and removed with no focus changes in-between, it is more
+    /// natural to activate the previously-focused column. This variable tracks that.
+    ///
+    /// Since we only create-and-activate columns immediately to the right of the active column (in
+    /// contrast to tabs in Firefox, for example), we can track this as a bool, rather than an
+    /// index of the previous column to activate.
+    activate_prev_column_on_removal: bool,
 }
 
 /// Width of a column.
@@ -1303,6 +1313,7 @@ impl<W: LayoutElement> Workspace<W> {
             active_column_idx: 0,
             view_offset: 0,
             view_offset_anim: None,
+            activate_prev_column_on_removal: false,
         }
     }
 
@@ -1315,6 +1326,7 @@ impl<W: LayoutElement> Workspace<W> {
             active_column_idx: 0,
             view_offset: 0,
             view_offset_anim: None,
+            activate_prev_column_on_removal: false,
         }
     }
 
@@ -1387,6 +1399,9 @@ impl<W: LayoutElement> Workspace<W> {
 
         self.active_column_idx = idx;
 
+        // A different column was activated; reset the flag.
+        self.activate_prev_column_on_removal = false;
+
         let new_x = self.column_x(idx) - PADDING;
         let new_view_offset = compute_new_view_offset(
             current_x,
@@ -1441,6 +1456,7 @@ impl<W: LayoutElement> Workspace<W> {
 
         if activate {
             self.activate_column(idx);
+            self.activate_prev_column_on_removal = true;
         }
     }
 
@@ -1459,6 +1475,12 @@ impl<W: LayoutElement> Workspace<W> {
         let window_idx = column.windows.iter().position(|win| win == window).unwrap();
         column.windows.remove(window_idx);
         if column.windows.is_empty() {
+            if column_idx + 1 == self.active_column_idx {
+                // The previous column, that we were going to activate upon removal of the active
+                // column, has just been itself removed.
+                self.activate_prev_column_on_removal = false;
+            }
+
             // FIXME: activate_column below computes current view position to compute the new view
             // position, which can include the column we're removing here. This leads to unwanted
             // view jumps.
@@ -1467,9 +1489,13 @@ impl<W: LayoutElement> Workspace<W> {
                 return;
             }
 
-            if self.active_column_idx > column_idx {
+            if self.active_column_idx > column_idx
+                || (self.active_column_idx == column_idx && self.activate_prev_column_on_removal)
+            {
                 // A column to the left was removed; preserve the current position.
-                self.activate_column(self.active_column_idx - 1);
+                // FIXME: preserve activate_prev_column_on_removal.
+                // Or, the active column was removed, and we needed to activate the previous column.
+                self.activate_column(self.active_column_idx.saturating_sub(1));
             } else {
                 self.activate_column(min(self.active_column_idx, self.columns.len() - 1));
             }